If you’re looking for cheap yoga, check out Mind and Body on 21st and Union. An hour of yoga there is just $5 and all classes are drop-in. I’m just beginning to practice regularly so I can’t speak for more serious students, but the instructors I’ve had have all been very good.
All ability classes are held Mondays, Wednesdays and Saturdays. An intermediate class is held on Sundays. Come early to this one though, because it can get quite crowded. There's also a number of pilates classes offered throughout the week.
Hardcore yoga types might be left wanting more challenge or number of classes. But, if you’re just beginning or looking to get a good stretch in, Mind & Body is the spot.