The price difference is negligible. You can get NiMH up to 2000mah at local electronics outlet for your TX. I still think you should buy an assembled pack rather than individual batteries. If you haven't bought them yet, or you can pay a little extra for a pack, you should do that. But I'm assuming that they gave you the 4 cell rx battery case so you can just pop them in, right? If that is the case, return all of them and get general use NiMH batteries. From what I know, NiCd batteries are very good for applications where you may need to operate at multiples of C such as for R/C cars. For rx and tx, that's not necessary as neither really require high currents. In fact, many are moving to Li-po only problem is the difference in voltage requires a step down.

Good luck!