$12-15k sounds like a lot when you say slab. That should include digging, footings, foundation , backfill then slab. Then it sounds not bad. But a home owner can usually do really well having this done by hiring the jobs out sepparately. The guy that gives you a price on the whole job either hires out parts of the job or has equipment sitting alot of the time that needs to be paid for. And then there are places in the mix when the homeowner can get in there and do some work. Evan if it's just leveling and compaction the fill.
You do want to be carefull about overbuilding for the hood. Most of us don't need trucks, campers, boats, cruises and seasons tickets but that never stops us.