Well, tiger, a lot depends on what your application needs. If all you need is to pass through everything that comes on serial port over the USB, your CPU need not do anything. Then you can use several cheaper versions (try Oki). If you want to connect the USB bus to your CPU (and your CPU does not have a USB controller), then the FTDI is a good solution at about $2.4 in 1000+ quantities.
You may want to consider the uPSD chip from ST Micro which as 8051 core with 256 kB flash, 32 kB boot flash, 32 kB RAM and a USB controller!!
Ajay