Please note you always need -some- memory - if you don't buy a derivative with internal memory you will have to attach external one. Add up costs of the memory chip, the latch chip, additional space and work with the PCB to attach them, taking up I/O port space, extra power to keep the external devices running, problems with ISP, and several more I either forgot or don't know about - it will probably be overall more expensive than internal flash version.