Can anyone breakdown the cost of developing and implementing an upper stage nuclear thermal engine?
After development, what could the price be of each engine produced?
Any sensible answer to your question should make an implicit assumption on the number of engines to be ordered overall. That is, we need to know the number of planned manned flights to Mars and other important destinations, since fixed (sunk) costs of an NTR-based program are significant. The following ideas are based on a few NASA documents discussing NTRs.
Let's see what has to be researched and done today (you can guesstimate person-hours and equipment expenses required, and multiply it by 2.5 to allow for your optimism):
There are a few concrete sums scattered across the NTRS, but I'd be wary of trusting them blindly. There are also many unknowns, from the number of firings required to the technology to manufacture fuel elements.