Mkelly is talking about solar panels, not solar covers, solar covers are just a blue bubble wrap that floats on the pool, retains the heat over the night time, a reel is between 200 and 500 bucks - that just lets you roll the cover up neatly.
Solar panels are black sheets that your water pumps through and dramatically increases the temperature of your water without burning up any more electricity or gas, it uses the sun's energy to heat your pool - a great investment and return on investment. If you have the room to place them or a roof to put them on they are the best! I would not even consider a gas or electric heater, matter of fact, I was advised against them by everyone that has owned one since the operating costs are ridiculous. Solar panels cost less on innitial purchase cost than a heater would too, so you save right away.