2003 Chevy Tahoe Coolant Leak?

Had low coolant message come on. Took the car for an oild change and asked them to top off the coolant. Ineed to call them and confirm if it was done because low cooant light is still on. This past weekend, I took the Tahoe to another mechanic for other work andmentioned the low coolant. They called to tel me they founda little coolant leak and detected the stuff on the bottom of the engine.

Questions:
1) Is it typical I wopuld not see the coolant on teh ground if there was a leak?
2) They told me it could be water pump, freeze plug, or the head gasket. Does this sound right?
3) Wat is typical cost for each type of repair?

Thank you.