I put my guitar in the trunk of a car a few months ago and I must've jammed it in too harshly (I know, stupid me.)

Turns out that I cracked the finish. When I brought it up to the mountains in PA, the colder climate cracked the top. I contacted a local luthier on an estimate for repairs and he gave me an estimate of about 100 + tax.

I've never dealt with a repair this big, so, is this a reasonable price?

Here's a picture:

sounds very reasonable to me. what kind of guitar is it?

Partscaster/Tele into a bunch of pedals, a Maz 18 head, and a Z Best cab.
you dont "have" to fix it. the crack looks like it runs between the grain in the lengthward direction of the grain. the actual wood grain has not cracked, but the space in between the grains has. Structurally, your guitar is fine. the crack shouldn't get any bigger.

but then again, it would be nice to get rid of crack. $100 sounds decent.