Hot tubs are not cheap and solar operated hot tubs are even more expensive. The usual hot tub can cost anywhere from $3,000 to $18,000 depending on how many bells and whistles you want.
Most people buy their hot tubs and expect to keep and use them for many years. For this reason, it is best to try to purchase as much hot tub as you can afford. If you buy a cheap one and then replace it in 2 or 3 years, it would have been cheaper to buy a more expensive one from the start.
Prices of hot tubs are determined by the size, number of water jets, number of pumps, number of features, and the overal design of the tub. You can buy hot tubs with stereos embedded in the sides, fancy lights, towel warming compartments, and a whole lot of other things. It is up to you to decide what you really want and need.
Some manufacturers allow you to customize your hot tub which is what you will want to do if you want a solar operated hot tub. You can find manufacturers that will sell directly to the public and you will need to find one that will work with you to customize your hot tub to make it a solar hot tub.