smell and windows fog when start up?

This is normally a build up of mould and mildew in the A/C heater vents, check that the drain tube is clear, and also check for moisture under the dash from a leaking wind screen seal, if it smell sweet it can be a leaking heater core, check these out first.

How much does this going to cost me?

To check the drain, is only probing the drain tube with a wire probe, the smell can be removed with commercially available vent cleaners that work very well, it should not cost very much at all if this is the problem, it will be the time taken that's all, i would expect no more than 1 hr,