Most INTJs impose a barrier of high expectations for their partners, since in all equity, they even more stricly obligate themselves to live up to these expectations too. Once they relinquish this suffocating need, they can start appreciating their partner for who they are, instead of what they should be/can be.
Intellectual stimulation is preffered or just a little bit of discernment, attraction, growth, authenticity, harmony, healthy ethics, and independence are things that INTJs put as a high priority. In exchange, their loyalty and the effort they put in to improve everything are unprecedented.
According to their subjective definitions, assessment of the parameters of the relationship, and their personal experiences, if these criteria are not met, and they feel stagnated, or worse, suffocated, INTJs walk away.
The cost/benefit analysis you're looking for is highly subjective though. Each INTJ has their own story, no, each person has their own story.