I believe I linked to both copies that included the MasteringBiology. The only difference seemed to be that US one might have a copy of the text as an e-book. I doubt making an encrypted PDF or equivalent merits the huge price difference.
Still your comment about the probability book is interesting. I wonder if this is particular to mathematics?
Here' s another example from Chemistry: Organic Chemistry by Bruice. In the US it's hardcover, in the UK paperback.
it's possible that the difference is the publisher. Coincidentally, the two books I list are published by Pearson who are headquartered in the UK. It may be they price their books for the independent markets, whereas US publishers are more likely to stick to one price? That's pure speculation though and we'd need quite a few more data points to figure that one out.