The usual case for cross-compiling is that your target is so woefully slow and under-powered that you would be insane to do anything else.
However, sometimes for one of the best reasons of all, "historical reasons", you might ship a 64-bit product but support building on 32-bit hosts, and thus cross-compile even on a very fast architecture like x86. How much does this cost, even though almost everyone is running the 32-bit cross-compiler on a modern 64-bit machine?
To test, I got a a 32-bit cross and a 64-bit native x86_64 compiler and toolchain; in this case based on gcc-4.1.2 and binutils 2.17. I then did a allyesconfig build of Linux 2.6.33 x86_64 kernel 3 times using the cross compilier toolchain and then native one. The results (in seconds):
32-bit
64-bit
6090
5684
6050
5616
6063
5652
average
6067
5650
So, all up, ~7% less by building your 64-bit code on a 64-bit machine with a 32-bit cross-compiler.