Look up Shax's Indispensable Haversack. Its not an item, but a collection of items from tons of different sources that are both cheap and useful. It contains references to tons of alchemical items and cheap magic items that have big effects like eggshell grenades and torchbug paste. It comes in 4 sizes, each cooler but more expensive than the last.
I dunno if your DM will allow much of the stuff in there, but it well give you a better idea about what to buy and use.