using the rule for kingdom building from the King Maker series and Castle cost 50 Build Points and a build point is 4000 GP. So 200,000 GP. A small keep would be 28 BP for 112,000 GP using the Garrison Building. The is also a Watch Tower and walls for about 10 build points. I took the watch tower and city walls but halved the city wall price as you aren't walling off 1/4 of a city but just going around a tower. So that would 40,000 gp.
The cost of a castle can vary greatly. Is the keep built of stone? How far away is the quarry? How many peasants were used to ferry it to the work site? They can't work all the time - or even most of the time - on the keep, as growing crops to keep them from starving will use much of their time. Only if you live in a place with high population can you afford to have peasants work continuously (excluding winter) on building the keep.
What about the local terrain? Is the castle built on a hill? What about the wall? Will it be a stone wall or a wooden pallisade? Will there be a moat (whether dry or filled with water)? How many stories will the keep be? Don't forget you will need to support multiple artisans for all the various work needed to build the keep.
You say it is a simple keep, so I am presuming only 2 or 3 stories with a wooden pallisade surrounding it. What about monster attacks during the building of it? If any goblin or gnoll tribes (or human bandits or outlaws) are nearby, I can see incessant attacks occurring both to raid for supplies and to try to prevent completion of the work. After all, once complete it will be quite a bit more difficult to drive out those soon to live within it. Wild beasts and even smaller dragons might also attack, simply because the work site is a concentration of prey (humans).
How magically advanced is the setting? Can you afford the cost of magic users using spells to hasten the work? Are their ruins nearby that could be used as a source of stone - or upgraded into a working outpost?
These are all the ideas that come to mind when I consider the idea of the cost of building a castle.
Thanks guys, that's a lot of good information.
Nyeshet, I would love to take all of that into consideration, I just don't know if I really have the time or ability to arrive at a good answer afterwards.
The keep in question is actually more of a moathouse surrounded by a stone wall. In fact, it already exists in a ruined state so my player was really considering fixing it up so that it's all new and usable again. The moathouse stands in a swampish area. Here's a pic of the ruined moathouse.