What you say is more accurate, but to make it a bit simpler you could also calculate:

A = Cost of Attack + Cost of Building
B = Turns left in the game * production gained from the land you are going to grab

Any landgrab is typically small compared the land the you already have, and does not dramatically decrease your tech level. This means that the gain per land of one grab is approximately linear. You would have to recalculate this per grab, and you also have to do a separate calculation whether or not to buy tech.