They're not more expensive! The advantage of a combo drive is that it can read and burn CD's faster, and can read DVD's faster.
For example, the NEC 2500A 8x DVD+RW/-RW Drive can only burn CD-R's at 32x, read CD's at 40x, and read DVD's at 12x.
The $54 (shipped from Newegg) Lite On SOHC-5232K 52X32X52X16 combo drive is simply a much faster drive...it can burn CD-R's and read CD's at 52x and read DVD's at 16x. It's cheaper than buying a seperate 52x CD-RW and 16x DVD-ROM, and is still $30 cheaper than the NEC 2500A.
In my opinion, neither choice is a very good one though. The NEC is too slow for CD burning, which is what I'll be doing a lot of. I'm waiting for a drive that can do it all: 52x CD read and -R writing, 16x DVD reading and -R writing, and dual layer writing. I guess I'll be waiting for a while.