Cost of head gasket replacement?

I have a 97 Camry with 150k miles on it. It recently started blowing white smoke out of the exhaust consistently. There is no noticeable loss of coolant or any other indicators, but I"m thinking it must be a head gasket leak. I'm not sure if it is smarter to just start looking for a new(er) car, or just get the head gasket replaced. Can you guesstimate the approximate cost to replace a head gasket on this car? Is it worth it with the high mileage? Otherwise the car is fine.
Sidrun