Just seen this over in the DVdoctor forums about a very cheap battery for EOS 300D - from 7dayshop:
http://forums.dvdoctor.net/showthread.php?t=36986
Eleven squids, would you believe? Though someone has commented that they bought them for a tenner last year!
Not sure what other Canon digital SLRs this might fit but this is what the site says about the battery:
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This battery is a high quality 7.4v (and top capacity) 1500mAh lithium-ion replacement for the following battery types - Canon BP-511, BP-511A, BP-51, BP-514
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Bob