I want to put some money into my garage. I want to heat the garage for next winter, and since the walls aren't insulated I figured I need to do this one fell swoop. I THINK I want to install a ceiling-mounted electric heater.

As of now, there are 2 15a circuits in the garage and only two receptacles. It's a 22x22" 2 car garage. The panel is installed right outside the garage.

Assuming I ripped the drywall down myself, any idea what it would cost to have at least a 240 circuit installed (for the electric heater), and a couple more receptacles on the existing 120v?

ALL it should cost is 10'-15' of Romex 12-3 and a good 2pole breaker and maybe a wall plate.

I punched through and put in a sub panel.  This is a fairly easy thing to do.

I then did a drywall cut near the ceiling to run Romex into my garage attic.  I reattached the drywall cut with wood blocks and drywall screws.  I made clean cuts and didn't patch afterward, in case I wanted to put in more circuits.

Drywall is easy to repair.  This job is even easier if you haven't drywalled yet.  I also did a run for a 220V compressor and 220V welder. My original15A garage circuit is on the same breaker at the 15A on the back side of my house under the patio.  Whoever wired my house had some good meth.

Other than running conduit along the outside of the wall, how do I get the romex to where I need the conduits? Just cut the strip of drywall to drill the holes to run the romex through?

cut a strip along the bottom. put your outlets in more then 4 feet high to stuff leaned against the wall wonts cover them

Garage outlets should be on a GFCI.
At least here they do. Im no electrician but I figure the same would apply to you as well

Hold on, I am lost. You're saying to cut a strip of drywall out along the bottom?

I assume this is an attached garage. Should make things easier. Mine was detached and required a sub panel.

A few things you should consider.

Put in more receptacles than you think you need. I made mine code as if it were a living space (within 6’ of any location on the wall).

Anything below 66” height needs tamper proof outlets. Check local code to see if yours is more strict or if they follow an older NEC.

First one on each circuit needs to be GFCI. The remainder can be standard.

240V isn’t difficult, but figure out the max draw you will want it for. By code, welding receptacles have a different standard. At the time they were my largest amperage device. I wired them up for true 50 amp service and now I can use one of them for my electric vehicle. Also figure out now if you need a neutral in your 240v.

If it were me, I would run two 20A 120v circuits new and one 50A 240v.

I would also consider natural gas for your heater or a mini split. Pure electric heaters are murder on your electric bill.

My garage is finished.  I just surface-mounted everything:  subpanel, EMT, and outlets; then pulled THHN back to the sub.

if you dont want to rip all the drywall out you can cut a strip at like 3-4" tall along the bottom so you can drill the studs for wire runs, then just put a strip of drywall back and hide it behind the baseboard molding without patching anything.

https://www.finehomebuilding.com/2004/12/01/running-cables-through-existing-walls

I bought a 240 vac 5K btu heater on sale for $79.  The circuit breaker,10/3 romex and misc $140.   This was purchased from the local hardware store so the prices are inflated compared to on line prices.

It will take me about 2 hrs. to install the heater and wire it up.