1 x 1GB RAM module is not the same price as 2 x 512MB RAM modules. I made that mistake pricing out an iMac with 2GB of RAM and couldn't figure out where the extra £600 came from.

On the other hand, if I'm not mistaken, the Mini only has one RAM slot. As a result, RAM is more expensive: you don't add RAM, you *replace* RAM; and big modules are more expensive than small modules.