You've got to watch out. On most of the planets (those that can actually support large pops, like Terran, Desert, and Ice) require at least 2 Pop Upgrades for you to break even. Also, even if your income through population puts you in the positive number, you are still generating negative figures for Under Development, even if you aren't losing money. So, remember this. When you capture a new planet, and you hit that Planet Upgrade, look at the underdevelopment line for the research info card. If that isn't 0, then you will still be generating underdevelopment fees after the first development finishes. You have to push the development until your development finishes off with 0 underdevelopment, then pursue other upgrades.
And yes, it does take time. Even while you are doing the development, you are still paying out money for underdevelopment until the work is done. So it's guarenteed that you will have underdevelopment figures. So, as theRetroboy said, because you end up with more planets, that's more sources of underdevelopment that you have to take care of. So, make sure, when you colonize, you immediately select the planet, open up planet development, and hit "q" (default for the top left selection) twice, and then worry about everything else. That way, you'll minimize underdevelopment costs in the shortest amount of time.
