Certainly don't know if it's apples to apples but I inquired what the cost reduction would be to LEAVE OFF my 6.5 x 6.5 attached spillover spa from my pool as I was spec'ing it out and was told the cost reduction would be approx. 6,500 dollars.
A spa can add 15k to 20k to the cost of the pool depending on lots of factors. We saved around 15k or 18k by deleting the spa from our pool build. For our build it deleted a heater, pump, automation and the extra gunite, coping, tile, etc associated with the spa portion of the pool.
We bought a standalone spa for $5700 the next winter and put it on a covered patio that ended up being inside a screen porch. Standalone tubs are more comfortable, cheaper to operate, always hot on standby, covered and all around better in my opinion.