Okay, so I just happened to have built a 24x30 garage starting last March. I'll give you a quick breakdown of how much its cost me, doing most of the work myself.
Concrete, monolithic slab, with associated dirt work and supplies - $9000 (Contracted Out)
One layer block set on the slab - $500 (Contracted out)
Wall Framing and Sheeting - $4000 (10 foot walls)
Roof Trusses - $4500 (6/12 Scissors)
Shingled Roof - $1200 (Contracted out the labor)
Engineered Siding - $4200
Windows x2 -$1300
Doors, 2 Overhead, One Man - $3000
Electrical -$1000
So about $29000 total
Now, my son and son-in-law have provided a bunch of free labor, they along with my Father-in-Law and 2 Brother-in-Laws helped set the trusses.
Now, could I have saved some, sure. Lose the scissors, 8 foot walls, cheaper vinyl windows, single overhead door, basic man door, vinyl siding. And wood prices have come down quite a bit, OSB was $22 a sheet last summer, is $15 now. Could I have done the shingles myself, probably, but a slip laying the tar paper scared me.