A company called zzSounds used to sell a 24" bass drum bag for $30. It isn't currently on their website but it looks like the stock comes and goes so it might come back (
http://www.zzounds.com/item--PRFPDBAG).
:!: However.... I have one that now sits in the corner collecting dust. It is just way too deep to be useful. The diameter is perfect but you could stack 10 beauty dishes in the thing. The bag was clearly made for a bass drum and not a beauty dish :shock: . It is so cumbersome that I don't bother to use it anymore. I even tried packing the dish, grids, Einstein, and accessories in there to make a one light kit but the bag is floppy and huge and cumbersome and it just didn't work out very well.
I couldn't even give the thing away - the local high school band director said he didn't want it and no takers (for free) on craigslist.
Wreath cases seem like the best low cost solution right now.