The scaling does seem a bit odd on assembly projects (getting the first upgrade, for example, gives a much better efficiency boost than all the others), but I don't know that it is necessarily a bug rather than intentional balancing. From my games so far, it is very easy to get planets with production high enough to crank out assembly projects of any level in a single turn, making higher level projects much more appealing regardless of the poor cost scaling. If that isn't the case for a specific planet, then I can always build multiples of the lower tech projects that give better efficiency, as long as it takes longer than 1 turn to build the one I choose (as production does carry over as I understand it). I could be totally wrong, but that's my take on it.