I'm assuming your pool is already in. You should get some prices on what a gunite spa will cost. When I had my pool constructed my builder told me $4,000 to add the spa while the pool was built. The price adding it after was 3 times as much as he has to bring out all the same subcontractors just to build a little spa. Do you plan to close your pool during the winter? If you do a free standing spa may be a better idea as you can leave that running during the winter. I wonder if you can get a freestanding spa that could be set in the ground? Access to the mechanicals may be an issue for this approach.