There is plenty of that already. An estate in a city (or another wealthy region) often pays more than the lordship of a rural/mountain/woodland region. I know that most of my characters have taken pay drops when leaving their estate to take up a lordship.
Beyond that, I can not imagine a sensible way of encouraging knights to not become lords when the opportunity presents itself. I am not saying there is none but I can't think of any.
Lastly, it sounded like the dev team was looking for a way to accomplish their goal of making knights a financial benefit to the lord (not just to the region and realm). They are pretty strapped for time so I figured I could try my had at finding a formula that might deliver what they were looking for. After all, implementing a formula might be less work than creating a formula then implementing it!
