I see a bit of inconsistency here, with so many postings putting the room importance high but room spend low!
I'd say the room makes up one-third of the total sound performance of a system.
Treatments in my lower level media room include carpet, realtraps at reflection points, heavy drapes on the front wall behind the speakers for absorbtion, and a new side wall at the front of the room for soundstage symmetry (the room was L-shaped, with the L opening to the side of one of the front speakers). I may still need more bass treatment and/or diffusion, but I really need to tune the room now. All my design consult has been DIY, using websites (including Rives), magazines and books, but that has its limits.
So far, my room spend has been about one-fourth of my system budget - but I know it's performance isn't as great as it could be yet.