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

If you want to not break the bank then go DIY. The advice to get Smith's book is good. Putting up cork everywhere is not!

All rooms need treatment because of overly long decay and to treat this the absorbers need to be thick. There are many articles online that will explain what's needed and why. Look up Ethan Winer's videos for a simple to understand intro.

Drapes and carpet being thin will only act as narrow-band absorbers and too much will ruin the sound not help it. The same clown is back again recommending artificial ficus trees 🙄

Room acoustics is the best and cheapest way to get really good sound. In fact it is the only way. The most sensible thing to do is download REW which is a free program that will allow you to measure your room's response and so identify the problem areas and also provide info on the length of time it takes for the sound to decay across the full spectrum. A microphone is needed that costs about $100 or less. You might also consider migrating over to a forum that deals more with acoustics like 'gearslutz'  It is taken seriously and you don't get the ridiculous suggestion to clutter you room with dust collecting ficus trees, for ficus sake.

I haven't read all the repsonses but some were right. The connection of your speakers to the floor is the number one culprrit. Sound travels faster through solid objects than air. You can try Iso Acoustics for that issue or build you own isolation platforms for the speakers

Yes interior acoustics will have benefit to tamming a reverberant environment which neighbours can hear, but it won't stop most of what they hear. Interior acoustics is mostly for your listening benefit.

REW is a tool, it does NOT tell you how to deal with acoustic issues, only experience & knowledge does. I've been designing rooms for over 2 decades and it has become a key offering in my audio business. Apart from buying some used acoustic treaments, there's no inexpensive way of getting a room to sound great.

effective materials cost money today even if you DIY. Learn about acoustics and what really works from a physics stand point, there are too many pretenders out there. Vicoustic seem to have the best price performance and WAF

Gary / audio by di tomasso .ca

It seems to me that your main concern is not upsetting your neighbours. Very honourable and most considerate of you . . BRAVO ! Cork will definitely not do the trick, The only real solutions - sorry but true - top class headphones OR sell up and move. Question - I wonder if your neighbours are as considerate as you most definitely are. VIVA LA MUSICA

If it is sound isolation you are after, you want to have a high STC rating (sound transmission class) for wall and floor construction.   
Currently the IBC required code minimum STC for a condo is 50.
An STC of 75 is preferable .
The primary ways of achieving a higher STC rating are mass, isolation &  attenuation.
I also live in a condo and am considering working on two walls, the wall I share with my nieghbor and the entrance wall to the corridor. Maybe with the construction of a sound isolating partition. But this takes space......

The best room treatment for me was done by accident!

My parents had a very dense hand-woven wool rug that they didn’t use anymore. Cost them around $5-6k and is beautiful so I didn’t want to get rid of it. It was nearly the size of my listening room so I put down a large felt pad on the wood floor and then placed the rug over that. It replaced a basic polyester rug that covered maybe half the floor. Immediately the sound softened and its was just the listener and the speakers.

Otherwise chairs and a couch in the room, but this rug seems to have stopped reflections very effectively.