The cost of the album, minus whatever you paid for songs on the album. If you bought an album completely for $9.99, and it costs $9.99 without DRM, you pay $9.99 minus $9.99 = 0. If you purchased ten songs for $0.99 each, you pay $9.99 - 10 times $0.99 = 9 cents to upgrade from ten songs with DRM to one complete album without DRM.
How do I find these albums in my library? I'm pretty sure there are some because I've spent over £1000 on iTunes .