I have a WaveMaster XXL bag in my garage gym. I got really lucky and scored a brand new one off of my local Craigslist for $75! These bags retail for about $250. My suggestion is to definitely check Craigslist. Another option would be to check with a local Karate/MMA/TaeKwanDo studio to see if they are selling any of their used bags. I went to a studio earlier this year, and they actually refreshed their bags about once a year. I asked the owner what he did with the old ones, and he said that he sells them for about $75.
I hope these suggestions help.
Best of luck!
--Fitgirl-ATX