Price vs value
Under good conditions, you should expect to pay $500 less, to $1 500 more for solar than for gas for the equipment, depending on the options you choose (automatic control system, and installation). From day 1, you will know how much it will cost to purchase and use your solar system for the next 10 to 20 years. Mechanical heating on the other hand is like signing a blank check. In any event, $700 is the average cost of fuel to heat your pool over one season, at 80 F, from May 15 to September 10, unless you choose to heat only on weekends!
If someone lures you to a solar system that costs less than $2 000, check to see if that includes PVC plumbing supplies (add $200-$400), installation ($350-$900), and the automatic control ($489-$600). Or it may also be a discontinued line! Panels range from $250 to $425 depending on the size. Keep in mind that some collectors are twice as efficient as other and require less area to offer the same heat, therefore reducing the overall cost.
The savings you get from choosing the least expensive system will be lost on your first claim for damages due to freezing, which happens to be the number one cause for damages. Coincidently, freezing is not covered by the average warranty. Your best buy is Heliocol, were it just for the freeze damage warranty.
On average you should expect 80 F or better from May 15, to September 10, five or six days a week, with a properly sized system, and use of a solar blanket. Remember, unless you get 3 references with pool set-ups like yours, you should only purchase a system that has been tested for Btu ratings by SRCC (Solar Rating Certification Corporation): how else can one determine if the salesperson is recommending the proper number of collectors?