It can be done with doing some yourself for fairly reasonable money. I have a 24x24 garage that's about 150 feet from the house that I did exactly the same thing. Keep in mind that I'm only using half the garage being a game room. Here's a list of what it cost me to build.

Divider wall (wood only) $178.00
Electrical with new 200 amp panel. $346.00
This includes 3 20amp circuits down 3 different walls
New ac unit for entire garage. $2476.00
Got lucky and knew a ac guy to help install. Paid him $500
Insulation for only the 12x24 side including ceiling insulation. $650
Sheet rock with mud and paint. $324.00
New double door. $689
I didn't keep all receipt but spent about $300 for locks, interior door and lighting.

Keep in mind that I did most of the work my self which can save a ton of cash. Installed indoor outdoor carpet months later but dont have a receipt for some reason. Good luck

I wish I would have looked at this before installed a standard split unit.