You can buy RS232/RS485 converters to adapt the kit radio DB9 interfaces, but you would need two (one for each radio) and I doubt you can buy one for under $50. They would not likely be useful for other applications that you have either.
Go with the Bluetooth setup.
If you are looking for an electronics project, consider basing it on the MAX3130 RS232/IrDA chip
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1947