When doing my 300' addition here 20 years ago, I got a good concrete contractor to do the foundation....wall on one end was about 6' higher than the other one, which was essentially ground level, good size hill for Florida....anyway the backfill was sand of course, he let it get rained on for a month to settle it all on, then did the slab......
total cost 3 grand for his part....whole addition only cost me 8 grand, then had to replace the roof with metal 700 bux.....
then I enlarged one slab outside the kitchen slider door, and built an additional patio.....I framed and screeded/finished it myself, had the crete pumped into the back yard....total cost on that was about 500 bux.....5" slabs....
hate to see today's prices......