How to find and suggestions for affordable personal retreats?

What are the best ways to identify affordable accommodations for personal retreats in nature? Any suggestions?
Ideally seeking affordable cabin anywhere in America for under $50 a night, or room with private bath and meals for under $75, and lower weekly rates. Prefer self-guided spiritual retreat with voluntary or no programs and peaceful natural setting for yoga, meditation, and creative inspiration. Please offer suggestions with website links to check them out.