We researched the indoor pool concept when we built. It didn't pan out due to the high costs of doing it as an addition. If we ever build a new home, it will include an indoor pool.
Some form of humidity control is essential for an indoor pool. What you have to consider is that whenver the air temperature is higher than the water temperature, evaporation kicks into high gear. 86 degree water is the norm, but air temps are more comfortable at about 84 or 82 in a pool area, with controlled humidity.
The only commercial system that I found when doing my research was one called Desert Aire. This company specializes in dehumidification systems and is the defacto standard according to several pool builders in my area. They are expensive, I was told even small systems start in the $6000-10,000 range (plus installation). So the price you are getting may be accurate.
We did the math using what we felt were reasonable numbers for water temp, air temp, and the number of square feet of pool surface to calculate an approximate evaporation rate. What we found was that without covering the pool we were going to need to remove quite a bit of humidity from the air to prevent excessive moisture and damage to the construction. As a step to reduce the size of dehumidification system needed we integrated an automatic cover for the pool. This inhibits the evaporation when the pool isn't in use, cuts down on the humidity, and reduces heating costs for the water. Based on what we felt were regular usage numbers for the pool (uncovered) we felt that regular ventilation during summer months, and portable dehumidification units during the winter that we would be OK.
Regarding the price she was quoted - seems terribly high, unless she's building an olympic sized pool area. I would think more in the $10,000 range for a new construction residential pool of say 400-500 square feet of surface area.
It's an expensive venture no matter how you look at it. Indoor pool need dehumidification, heating and cooling (although dehumidification usually provides adequate heat), proper building materials, special engineering of the structure, and speical building code consdierations. Done right, the pool and enclosure could cost significantly more than the rest of the house.
