I think this should be allowed.
When you add walls, paths, and other features to a town, other than players houses, the amount of space can be quite limited, if trying to avoid making the town looking to close an squashed together. This would give a solution to the problem of mayors wanting to sell larger plots of land.
I don't know about actual prices, but, I think the more bonus plots that are bought, the more expensive they should become. This means that most towns will be able to afford a few, and then the towns that have residents that regularly deposit money to the town bank, will be able to afford more.
As for a limit on the amount a town could buy, if the price of a bonus plot increases as more are purchased, eventually each town would hit their own limit, as they wouldn't be able to afford any more until they gained more money. However, people such as Nbord have loads of money, so maybe there should be a limit...