This is gonna sound like a bizarre question, but it's almost entirely for Tyranid players, but what's a decent cheap (model wise) army that can be assembled? I'd like to start playing this but my money is...limited at best so I want to make what I get count.
I'm not sure how you mean this - do you mean you want a cheap Tyranid army, or just any army in particular?

In the case of the former, looks for second-hand armies.

I have $300 worth of Wraithguard and Wraithlords in my Eldar Army, for which I paid about $170 and then another $10 for a bottle of paint remover. Even if you want to buy unit-by-unit, it's much cheaper to do, though occasionally you can find a really good "2000 for $50" deal if you know where to look.

I recommend eBay as a starting point, and what you should look for are eBay Shops that sell plastic kits without boxes. These are usually at around $10 cheaper than buying new, or even from another shop that sells them boxed.

Not a particularly good way of supporting Games Workshop, but quite frankly I think they can look after themselves while I pass on my own wisdom to new players.

In the case of the latter, however, an all-plastic Tyranid force is never going to be the cheapest of armies, due to sheer size needed and the monetary expense of the larger, points-eating models.
Probably the best thing to do is buy a couple of of the Boxed sets, which include several different types of 'Nid both big and small, and are generally a better deal than buying each unit separately. Just be sure to find out what is in each box and then write out an army list with those figures in it to be sure that nothing goes to waste, and it probably won;t be too hard or expensive to hit a decent sized AND effective army.