Rigging a live-capture trap shouldn't be difficult
The Arms and Equipment Guide describes the cage:
A typical cage consists of a solid metal or wooden top and bottom, solid metal bars equally spaced around the sides, and a door. It has a latch, but any lock must be purchased separately. (21)
Cages come in Diminutive (10 gp; 13 lbs.), Tiny (15 gp; 25 lbs.), Small (25 gp; 75 lbs.), Medium (50 gp; 100 lbs.), and Large (75 gp; 200 lbs.). Most cages have hardness 10 and 30 hp (15 for Tiny cages and 10 for Diminutive cages). I assume Fine cages are jars. Serious trappers make cages of ridiculous materials like riverine (Stormwrack 128) (2,000 gp/lb.), which withstands anything up to a disintegrate effect.
Ideally, all one need do is buy a cage, tie a rope to the door, put some food in the cage, hide some distance away, and ready an action, setting the action as pull the door closed and the condition after the creature enters the cage. This does, however, require a creature to wait nearby. Untrained hirelings cost 1 sp per day. Such a trap should work on monstrous crabs and even some orcs but is less likely to work on roadrunners. Plan appropriately.
If further game mechanics are necessary, there are a few more choices.
- A booby trap (Dungeonscape 54) drop trap has a base cost of 1 sp and take 1 full-round action to deploy for the prestige class trapsmith, a standard drop trap dropping a heavy object and an advanced version dropping a very heavy object on the victim, the latter explicitly including a net. The Dungeon Master's Guide II also lists booby traps, there costing 50 gp or free with a successful 10-min. Survival skill check (DC 20) and taking a successful 1-min. Craft (trapmaking) (DC 20) check to deploy, but those don't include dropping things on folks (but they should).
The Dragon #295 article "Building a Better Rogue Trap: New Traps to Bedevil Invaders" supplies pretty much exactly the kind of trap you're looking for but built into a stronghold:
Cage-drop Trap: CR 3; mechanical; location trigger; manual reset; Reflex save (DC 25) avoids; confinement; Search (DC 25); Disable Device (DC 25). Market Price: 13,500 gp. (60)
Earlier in the article, if you'd rather build it yourself than buy it, the cage-drop trap's Craft (trapmaking) DC is listed as 20, and its raw materials cost as 4,500 gp. Obviously, the cost is excessive for a Wile E. Coyote-style, ✖-marks-the-spot, out-in-the-wild style trap.
Races of the Dragon introduces folding traps, magic items that are, essentially, portable traps. One of those is the CR 3 ceiling pendulum folding trap (28,200 gp; 1 lb.). Although the text says, "Other varieties of folding trap might also exist, but they all must involve the use of mechanical traps that have an automatic reset" (124), yet automatic reset adds only 500 gp to the price of a cage-drop trap, still less than the 14,100-gp ceiling pendulum. Cost for a folding trap (cage-drop trap) should then be similar.
Races of Faerûn revises the footsaw trap (700 gp; 15 lbs.) from a Dragon #285 article about halflings generically (so it's not like such traps are only ever found on Toril or something):
This item is similar to others of its kind that are typically designed to trap large animals such as bears or cougars. Its jaws are fitted with thin saw blades mounted on springs, and the mechanism is designed to cut the feet of any captive who struggles against its grip. The ghostwise hin did not invent this trap, but they have made excellent use of it: When enemies threaten, they salt the ground near their campsites with these dangerous devices, hiding them under thin layers of leaves or soil. A creature can discover a concealed footsaw trap with a successful Search check (DC 15). Once found, the trap can be disabled with a successful Disable Device check (DC 15).
Anyone who walks over a footsaw trap triggers it; the trap makes a melee touch attack with a +8 bonus. If its attack is successful, the trap deals 1d6 points of damage to the victim. A victim caught in a footsaw trap can move at half speed if the device is not attached to another object (such as to a tree or boulder by a length of chain). If it is attached in such a fashion, the victim must break the attachment, otherwise he cannot move. The trap inflicts an additional 1d4 points of damage from the saw blades every round that a trapped victim takes any action that involves movement. A trapped creature can pry open the jaws of the trap and escape (Strength check DC 25) or loose himself with an Escape Artist check (DC 30). Failure means that the trap deals another 1d4 points of damage to the victim and that the victim remains trapped.
A footsaw trap is a CR 2 trap. They can be constructed with the Craft (trapmaking) skill (DC 20). (159)
(The revision lowered the original's Search and Disable Device DCs by 5.) A DM might be persuaded to allow a skilled craftsman to create a less lethal version. This, too, seems excessive by comparison. For instance, for the cost of a lone footsaw trap, one can buy 8 Large cages and for, like, 2 months hire 16 untrained hirelings to stand watch and still have gp left over for rope and bait.
If more money than sense can be devoted to the task, magic can be employed. Instead of equipping untrained hirelings with mere nets and saps, they could throw buckets of cash: beads of force (DMG 248) (3,000 gp; 0 lbs.) capture for 10 min. creatures that fail a Reflex saving throw (DC 16) and iron bands of Bilarro (DMG 261) (26,000; 1 lb.) with a successful ranged touch attack capture a not-too-strong, not-too-slippery creature indefinitely. Likewise, a serious lone trapper may, for example, employ the following:
- The metamagic feat Earthbound Spell (Player's Handbook II 91) for 1 hour/caster level creates a magical trap from a spell the feat's possessor casts that's triggered when a creature steps on a nearby square. Any of the decent wall spells should be sufficient as would something like the 3rd-level Drd spell dominate animal [ench] (PH 224).
The mirror of life trapping (DMG 262) (200,000 gp; 50 lbs.) says
Any creature coming within 30 feet of the device and looking at its own reflection must make a DC 23 Will save or be trapped within the mirror in one of the cells. A creature not aware of the nature of the device always sees its own reflection. ... When a creature is trapped, it is taken bodily into the mirror. Size is not a factor. ... If the mirror’s owner knows the right command word, he can call the reflection of any creature trapped within to its surface and engage his powerless prisoner in conversation.
Which, when used for this purpose, makes the mirror also kind of like an interactive National Geographic channel.
But, really, unless the trapper's made of money, the target is extremely high value, or the campaign is based on capturing monsters alive (then, perhaps, training such monsters to fight each other), a couple of commoners and a cage should be enough.