Training up a village or town is almost never worth it and here is why:
Yes, at maximum quality a village can spit out about 3k denars a week. I found out that if you have everything maximized and made sure every single villager made it to their destination, you can get a destitute village up to the maximum(Opulent I think?) in something like 2 months (7-8 paydays). The problem is that you must be able to maintain that growth unopposed and as soon as war breaks out, all of your investments go to waste. When a lord 'visits' your village, the wealth level drops almost immediately, even if they do not succeed in completing the pillage I have noticed that the village wealth drop a level immediately afterwards. Also, war means more bandits and less patrolling lords, if enough villagers gets intercepted the wealth level will also drops. So unless you can be sure that your village will not get raided AND you can consistently patrol it with 3+ patrol groups (it needs to be small, medium and large, with the largest capable to dealing with small lord parties to ensure that the patrol is effective. If you just place one or two large patrols they wont' be able to chase down anything). Finally, there is the fact that your patrols do not extend far enough to protect your villagers the whole way - which means they could still be waylaid despite your best efforts. Of course, patrols are not free - I find that I need to replace my patrols about every 2 months or so, even if you only fill it up with medium level troops, you still need to spend a good 1-2k denars on each patrol just to make sure that they don't die (it's another 1k if they do).
And then there is the upgrade system: total cost of all upgrades is 126,000 denars for a village(157k for a town). The income difference between a destitute village and a opulent one is about 1500-2000 denars/wk(about 3k for at town, my poor as hell towns usually earns about 3k apiece, while I've seen 8k or higher for opulent ones. however, towns don't usually drop to destitute even if you ignore the hell out of it, trade caravans are harder to intercept than villagers). the payback period then is between 84 wks and 63 wks: 588 days to 441 days. It means it takes nearly an entire campaign to offset the cost of your village improvements before you make a profit - assuming all else goes perfectly. Take into account build skill and you can cut that down to about 1/2 (or 1/4 if the PC actually invests in engineering) - still around 100-200 days at the absolute BEST cast scenario. In comparison, the most efficient enterprises cost between 10k-12k denars and yield about 400-600g a wk, with a payback of 20 wks. This is nearly 10 times more efficient than the best possible scenario for a village. Of course, these improvements are not 'build and forget', if your village gets raided OR if your town gets besieged, they have a chance to get destroyed, and must be repaired at a hefty cost. Pay back times are also significantly longer if you let your villagers/trade parties get intercepted, the calculation is made as if you complete your construction all overnight and the wealth level immediately skyrockets to opulent. It is almost impossible for a town or village to payback the investment you've made into it during the course of a normal game, at least not doing so in a worthwhile fashion.
Keep in mind that the effective return rate of a town infrastructure upgrade is somewhere around .5% and 1% per week of invested time. The question you should ask then is this: can i get more out of my denars by buying 'useful' things like troops and equipment than .5-1% rate of return? If yes(and this should almost always be yes), then you do NOT invest in villages.
Let your villages burn, let the bandits rape and pillage your trade parties and let your kingdom fall to ruin. Because lets face it, it simply isn't worth your time to care.
