Prices and weights for Dungeons & Dragons 3.5 weapons bigger than Large and littler than Small are in the Rules Compendium. On Weapon Qualities on Cost says
This value is the weapon’s cost in coins. The cost includes all miscellaneous gear that goes with the weapon, such as a sheath. This cost is the same for a Small or Medium version of the weapon. A larger version costs twice the noted price per size category larger than Medium. A smaller version costs half the price per two size categories smaller than Medium (round up). (151)
And on Weight says
This is the weight of a Medium version of the weapon. A larger version weighs twice as much per size category larger than Medium. A smaller version weighs half as much per size category smaller than Medium. (ibid.)
This is—sadly—completely different from, for example, the Player's Handbook chart Armor for Unusual Creatures (123). (I used that chart to scale weapons using the Humanoid cost and weight columns before discovering this information buried in the Rules Compendium.)
Thus, after this DM's PCs defeat a typical titan (Monster Manual 242–3), they'll find that titan's +3 adamantine Gargantuan warhammer has a cost of 21,096 gp and a weight of 40 lbs. (That is, it's 8× the cost and weight of a normal warhammer due to the Rules Compendium's rules and an additional +3,000 gp for the special material adamantine (Dungeon Master's Guide 283)—but see below—, and +18,000 gp for the magical +3 enhancement bonus to attack rolls and damage rolls (222). Note that the cost of the warhammer's masterwork quality is subsumed in the cost of the special material adamantine.)
Do special materials' flat costs and weight adjustments scale?
So far as I can tell, the rules are silent on whether to scale the price and weight modifications due to special materials with a flat cost when those special material are applied to weapons. (For what it's worth, the D&D Main FAQ addresses this obliquely for armor on page 50 yet makes no mention of how or if to do the same with weapons.)
So you know, this DM does not scale special materials' costs and weights: this DM believes such scaling unfairly rewards the little and punishes the big, and leads to every big creature (and some little creatures, too, depending on the special material) buying a Fine magic weapon made of a special material and adding to that weapon the magic weapon special ability sizing (Magic Item Compendium 43) (5,000 gp; 0 lbs.), bypassing most exorbitant costs a big weapon would normally incur anyway. It may not be particularly (ahem) realistic to have Fine weapons cost the same to be made adamantine as Colossal weapons, but it makes this DM feel better to charge everybody that flat +3,000 gp rather than charging for an adamantine Fine dagger +750 gp and for a Colossal adamantine dagger +48,000 gp.
Also note that this DM also rarely uses areas of antimagic—which would temporarily negate the magic weapon special ability sizing—, so in campaigns where antimagic is more common, scaling the cost and weight adjustments for special materials and adding the magic weapon special ability sizing is a calculated risk that may sometimes leave a wielder with a favorite weapon bought on the cheap that's not as useful as the wielder would like.