Prices (UK) for the sealed 6204 bearing seem to range widely, with some vendors providing details of quality level while others give little information. Branded vs. unbranded is the big differentiator, although branded product prices vary widely too. I've seen 6204s at GBP1 (yes, one) and I've also been asked for GBP11 per bearing ("the list price is GBP44, sir"), with the main cluster of branded (SKF, Timken, 'Japanese' brands...) prices in the GBP4-6 range on a popular auction website.
In conclusion, shopping around looks essential and I think I'll be picking a branded product somewhere in the middle of the price range.