A hovel
20 coins
A cottage
500 coins
A house
2,500 coins
A mansion
50,000 coins
A keep
75,000 coins
A castle
250,000 coins
A grand castle
1,000,000 coins
A month’s upkeep
1% of the cost
Given how much I've written about A Whole Damn Army there's probably enough there to make some assumptions about what kind of fees are charged by legitimate road authorities, and The King's Tax Collectors. A good baseline is that the highest ranking feudal figure in the settlement's home determines the monthly tax based on a value roughly twice the amount of a month's upkeep (so a city supporting "A Grand Castle" has up to 20,000 coins in its coffers.)
Resources
This article suggests that a "An orc warchief’s tribute” is reasonably 1 point of Resources." and that each extra point of resources is an order of magnitude (roughly a factor of 10). That means the Grand Castle settlement is worth ~3 Resource, the Keep or Castle settlements are worth 2, and the Mansion at 1000 coins is worth 1 Resource.
Taxation and Population
If we assume the majority of a population lives in cottages, and half of the upkeep is taxation, that amounts to a tax of 3 coins per person. This means "A Grand Castle" needs a supporting population around 7,000 (6,666 to be more specific), a Castle needs around 900 (call it a thousand), a Keep needs 250, and a mansion 200. So...
A Village is less than 200 people
A Town could reasonably be 200-500
A Keep is probably 500-1000
A City is 1-10,000, with the largest medieval city ever being no more than a million or so (It would take less than 50,000 to make "A dragon's mound of coins")