A solar cover is sort of a misnomer, they do little to capture heat, instead what they do is help prevent heat loss by evaporation, which may help some with your goal. Price wise a solar cover is going to be MUCH cheaper than effective solar heat panels, they are also not just an either or, but work well in combination. However if budget is a problem for a heat pump, initial purchase cost of a quality worthwhile solar heater system is not going to be all that much if any cheaper, assuming you have a good place to put it.
You can expect a solar cover to be available mail order for under $100, a solar heater big enough to make significant difference in your pool will need an area of at least 50% the surface area of your pool, or about 280 sq ft. Or put another way about 6 4x12 standard solar panels. A basic DIY installation kit from one of the major mail order vendors (solardirect) for 6 4x12 panels, and associated roof mounting hardware, etc. would run about $1771 without solar controller or about $2144 with basic solar controller, plus probably another $300+ in incidental plumbing, this assumes you would need a larger pump.
p.s. oops it looks like my math was off, but not by too much, it looks like 3 or maybe 4 panels would be more approriate, so knock off about $720 from those prices.
