Momonga-sama covered the more standard items, I want to delve into the often overlooked items, that just require the right tactics - and often some preparation - to use.
Divide et Impera - Divide & Rule
Wood, Handaxe, Hammer & Nails in the core weapons, adventuring kit, estimated from trade goods (time, 6gp, 5SP, 5sp/lbs (?))
For a mere 10 gp, the adventuring party can have enough materials to erect a few barricades - which is very useful to either block off areas or to seal off one tunnel or another, so the group can use mass tactics against the incoming undead: by forcing the undead to pass through a 5-feet long, 5 feet wide funnel, a group of 3 can attack the single undead in the zone with only getting a single user's attacks back. if the middle one retreats one step, it is a 5 against 2 ratio.
Shovel in the adventuring Kit (2gp)
With a shovel, the group can (again spending time) create more sturdy barricades from earth - or at least create difficult terrain for the incoming hordes. With a pick or axe in addition, this can be used to clear terrain for the preferred combat zone too
Tanglefoot bag in special Substances and Items (50gp)
Riot-gel... if the undead come in too fast, slow them down, glue some of them to the floor (and thus hinder the advancement of the others!). Also nice to use on creatures in the "kill zone", as it reduces the attack capabilities as well as dexterity, thus reducing the AC a bit.
Kill it with Fire!
Lamp oil & Pick in the adventuring kit. 1sp/pint, 3gp
Kill it with fire is pretty damned universal: while it might not harm the enemies too much, setting some zone ablaze does 1d3 fire damage for 2 rounds - which is not much, but if you have a smooth stone floor and a bit time to prepare the field, you can dig a grove to pour the oil into to keep the fire alive much longer as you refuel it from a barrel: let leak 1/2 pint of oil per square over the thin channel and you can keep the fire for some time... Downside is, that it is heavy and it needs preparations.
metal sheets/basins/channels estimated from trade goods (3sp/lbs(?))
With some metal sheets and a hammer to shape them, one can prepare. Or, as those are not crafters, get pre-formed flat 5x5 foot basins & channels. This makes setting up a fire-trap a manner of minutes - add in some of the nails to fix them to the ground and they can't be tossed away that easily. This also allows using the fire tactic on a soaking terrain. Also, one can easily use the basins to fortify the structures to channel the enemies - plate the wooden wall with metal sheets!
Alchemist Fire in special substances & Items (20gp/flask)
Where lamp oil is not enough, refined Alchemist's Fire can help: it has a 5feet splash zone and deals 1d6(+1d6) direct/1 splash damage - not much, but enough to bolster up the defense. In addition, just hitting anywhere, where the basins/channels are, will ignite the oil in them, switching our fire-zone to on.
Well, we try to kill em with fire already, amp it up! Let's get something actually explosive! There are rules for guns! I couldn't find my DMG at first, but now I got it again: p 145 holds guns, and so it does gunpowder!
Gunpowder: While Gunpowder burns (1 ounce consumes itself in 1 round and illuminates like a sunrod) or even explodes in the right conditions[...] Gunpowder is sold in small kegs (15 lbs capacity, 20 lbs total weicht, 250 gp each) and water resistant powder horns (2 lbs capacity and total weight, 35gp[...])
15 lbs for 250 gp a barrel. We don't need cartridges, we want barrels or bombs. Bombs detonate for 2d6 fire damage in a 5ft area, weigh a pound and can be thrown in 10 feet increments. For a mere 150 gp.
The barrel is perfect to just place around a corner, shoot at with a flaming arrow and go to cover. Or light a fuse that goes into the barrel and roll/hurl it into the undead horde! Better even: sick a flask of alchemist's fire (special substances & items, 20 gp) into the powder, and the group has an impact triggered explosive. Better get a strong person to hurl it far enough... And ask yourself how much damage 15 lbs of gunpowder do, because there are no rules for gunpowder barrel detonations. It is most likely enough to bring down a house or even the ceiling in a fortified structure: A book indicates that 1-4 lb powder charges were used commonly for canon veuglaire, the main group seeming to be the 3-4 lb division. However, no caliber of those is given.
Not enough? We can get Dynamite! 3d6 bludgeoning, 5feet blast zone, and they are stackable... no price given though...
So, take your time and use it to make the ceiling come down in those dungeons. Crushing damage from ceilings is far than enough to rekill most things not incorporal.