How much does it cost (parts), and how hard is it to fix, assuming I have decent mechanic skills?
Example just for the thermostat housing re-gasket (Autozone prices):
- Gasket: $1.50
- Sealer: $4
- Antifreeze: $12
Additional if you want since you're in there:
- Thermostat: $5 ~ $14
- Upper hose: $10
- New clamps: $5 ea?
Outright it's not a complicated job. Just keep in mind that sometimes it is difficult to get the thermostat housing to seal properly, so any repair needs to be done carefully to avoid the need to do it over - be sure both surfaces are clean and flat.
Also getting the thermostat to lock in place and stay positioned during installation can be tricky - pay attention that it is not in backwards.
Getting the bolts out if they are stuck can be a chore, especially if one breaks during removal. Be sure to clean the connector and post on the temp sender.
If the housing is badly rusted, warped or bent, it might need to be replaced. Sometimes you can source a good one here from another owner. Pricing depends on them. I think last one I sold was $35 plus shipping.
In your case, it might just be a loose clamp at the upper hose to the thermostat housing, so don't make any plans/decisions until you know exactly where the leak originates.
Please keep in mind that small leaks on our cars are usually a symptom of the cooling system needing attention overall. If the thermostat housing is rusted, as an example, it might mean that other components are equally 'aged' and in need of replacement.
By the time you replace the radiator, cap, thermostat, water pump, hoses, belts, gaskets, heater core, coolant etc. things add up - our cars depend heavily on a correctly performing cooling system, and like any older vehicle, things don't last forever, so plan repairs accordingly and good luck.
Ken