The trick to room treatments is to not use too much absorption. It's almost impossible to use too much diffusion.
Unfortunately, good diffusors are much more expensive than absorbers. There are inexpensive diffusors available, but they are not anywhere near as effective as good quadratic residue or skyline diffusors. You basically need diffusors that are at least several inches deep (ideally 6" or more) to achieve any diffusion at midrange frequencies.
My room is 17' x 29' x 8' with the speakers along the narrow wall - about 7' from the wall behind and about 4' to the side walls. My walls are mostly cinderblock with stone floors, so without some absorption, the room was super live. But initially, I overdid it a bit.
I used GIK absorber/diffusors behind the speakers, and these proved to do too much absorption and not enough diffusion. I replaced these with ATS quadratic residue diffusors, which significantly improved the liveliness and sound stage.
I have dipole speakers, so my treatment approach may not be as effective for box speakers.
In addition to the QRD diffusors behind the speakers, I have some 2" absorption panels on the cinderblock walls to the side of the speakers. A total of six 2'x2' panels on each side covers about 25% of the wall area in the front 10' of the room. I'm also using DIY tube traps in the front corners (behind the speakers).
I have a ceiling cloud using six GIK 242 panels (2'x4' with about 2" of absorption material) and a large wool area rug. Since I'm now using line array speakers which fall off fairly rapidly in the high frequencies above and below the speakers, these floor/ceiling treatments don't do as much as they did with my previous point source speakers, but help keep the room reverberation time to a reasonable level.
The side walls closer to the listening position have GIK art panels with scatter plates. These do a modest amount of diffusion and absorption.
Overall, I'm very happy with the results at this point, but I did do a bit of trial and error to get it dialed in.
My total investment in room treatments is about $7K, so not exactly budget, but I have a fairly large room and purchased commercial products for everything except the corner bass traps. You can certainly save a fair amount by building some or all the panels yourself.
You can see my room treatments in My Virtual System page.