Most of the time - i.e. if the decryptor gives improved invention chance or additional runs - you are balancing the cost of the decryptor against the cost of the datacores needed to achieve the same result without the decryptor.
If I am inventing T2 ships or rigs - which are 1 run BPCs - I will use a parity decryptor that gives me a 50% greater chance of getting a 4 run BPC. In this case the datacores to get the same number of runs without the decryptor would cost more than the decryptor.
For modules and ammo, you get 10 run BPCs and the extra runs/invention chance provided by the decryptor is rarely worth it.
For T3 the relics and hull sections are very expensive and I use optimized attainment to maximize probability of success.