When you Update the Researchcosts via SQL automatically it gives negative entries, but as far as I know if you do it manually for each tech it should be fine, I will test it
Civ4 was no problem with research costs of 250k and more.
Edit:
Nope not even manuall entries for Techcosts above ~200k are well calculated
Thats the reason CCTP converted back to 1's system from 5's system, it happens with Production/Gold and Research, there seem to be a hard cap and once you reach or go over this it will revert to -1, resullting in 1 turn techs/buildings/units.