When you go late by 8 days, they charge you the full "selling" price of the movie minus the rental fee. If, within 30 days, you return it, they give you THAT amount back minus $1.25 "restocking fee."

Once you hit 37 days, you own it. It's not a late fee. It's $25 to buy. Which is the price, I presume, that it was selling for when you hit 37 days. If it was then selling for $15, then this should have been what you paid.

See http://www.blockbuster.com/stores/membershipTerms

You may feel robbed, but these are the terms.

That said, "any time" does mean "any time." Even beyond 30/37 days. So, I'd suggest that you double check to make sure that there aren't additional sentences, like what's on the web site, that say that this applies only up to 30 days.