Don't know if that could be squeezed in your budget, but you can grab ARC for $179 at Audio Deluxe.
Simple things like bookshelves (w/ books in it obviously) can help. Me, I've learned to use and recycle any and everything to at least help improve the acoustics of the rooms I have to work in. My next challenge is to improvise a "vocal booth" w/ what we have here, to record some vocals for a project my wife is working on.
Ultimately, what you can or should do for treatment depends on the room you're working in. Shape, ceiling height, etc... There isn't really a formula.
You may want to grab a copy of Mike Senior's "Mixing Secrets for the Small Studio" - available on kindle reader for pocket change. It may give you a hint on finding what you should focus on - and therefore, how you could use what you have at your disposal.