$25 per sq to install
To rip out start over I say at least $50 per sq But this shooting in the dark hard to say with out looking at it in person
20 man hrs seems low but I think you will eat the 20hrs up on the rip out and leveling part then you have install and sweeping in the sakrete takes time
Any photos of it
Do you have all the equipment to do the job or you going have to rent something
Snyder's New Lawn Service Inc.