I just bought a 25-pack of BD25 for $9.99 at local Micro Center. But BD50s are still average $4 to $8 per disc there.
Aside from the bitrate issues I encountered before, you will also be faced with Cenaiva problem. That's why I ditched the idea of backing up my BDs to a physical disc. Not only it costs too much but also won't work most of the time. Instead, I will just keep the mkv files with HD audio intact. The Sony player I have can play MKVs with HD Audio via disc, USB or streaming without problems. Better yet, Cenavia protection does not apply to any of these usages, only to BD movie discs.