What do you plan to do with the garage door? That's where the heat/cooling is lost.

Unless there are inlets/cold air returns for HVAC in a room, insulation does no good. And adding such venting to the garage now would be pretty expensive.

Plus the problem Christopher_H mentioned.

Thanks for the comments. Garage door is insulated. If I let the inside door open that's attached to the house, wouldn't it let the ac air or heating flow in?

The door is insulated but it is still basically a sieve, and opening the inside door will not give enough of either to do anything more the run up your HVAC bills. If the Insulation Contractor (who makes money on sales) says not to do it it is a waste of money that is pretty revealing in my opinion.

Can return and supply ducts to a garage in a way that does not violate building code? I thought there were fire barrier issues.

For air to flow, there has to be a path consisting of an inlet source and an outlet source. Opening a door on one side does not make a complete path.

Most garage doors have a gap around the edges of the door, which would pretty much make conditioning the space impractical.

So not a very good idea, huh? Two contractors quoted $500... I thought it was a great idea... Apparently not. Thank you for great input, guys!