How do you enhance a bad listening room's accoustics without breaking the bank? thoughts?

Just placed an order from GIK.  45 days out for my hybrid diffusion/absorption/trap panels.  My wife approved them as they are quite attractive.  

It's really hard to insulate, but relatively easy to correct. 

To insulate you need different construction methods.  Double drywall, resilient channel, etc.

To improve, bass traps and panels.

The good news is that if you make your room sound good you'll be able to play at lower volumes.

Check out GIK acoustics and ATS for effective, high value solutions and professional advice.

  Glad to say that most of the responses here are good ones. So often with audio, we simply have to deal with what we are dealt for listening. In your case, you have two issues. I agree very much with getting things right from a listening perspective first by way of adjusting listening position as well as speakers, etc. i found something very simple for me in this way though. Pillows are effective for some of the reflections in my room. I wait until the wife leaves. heh heh, then I use dining room chairs with bed pillows placed them. It doesn't take much in the way of placement to hear a really good difference! No, it doesn't solve all problems, but it really helps with first reflections. Isolating speakers and subs for the benefit of the neighbor may be beneficial as well. Sound travels through flooring as well as walls. At least bass does. Good news is that when you arrive at a better sound in the end, likely you will not be listening as loud. That is the case in my experience. Cork on the walls maybe good on the front wall, but I am thinking that the side walls may be too much of a good thing. 
 It sounds so generic, but yes, you are going to have to experiment with different placement, room treatment, and more before it is all over with. 
 One responder said to Know The Problem before you attempt to remedy it. Now THAT is good advice.

If handy, you could purchase Owen Corning Panels and cover them with your favorite fabric. This approach will save you lot of money.

Before you leap into solutions a quick review of Get Better Sound by Jim Smith is worth a try.  Plus, its very inexpensive to set up a 30min conference with Jim; he's very nice, patient and helpful.

He will ask for a diagram and pictures of your room.  He's equipment agnostic; he'll focus on placement and potential room treatments.

Best money I ever spent!
https://www.getbettersound.com/