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You might think that a costly sunblock would be better at blocking UVA and UVB rays than a store brand at, say, Walgreens. However, according to Consumer Reports' annual sun survey, that's wrong.

In tests of 12 sunscreen lotions and sprays to gauge their effectiveness in protecting against UVA and UVB rays - both of which can cause skin cancer - six sunscreens rated "very good" overall.

Three of the four top-scoring brands came from national chains Target, whose Up and UP Sports SPF 50 had the highest overall score; Walmart, whose Equate Ultra Protection Sunscreen SPF 50 was No. 2; and Walgreens, whose Continuous Spray Sports SPF 50 was No. 4.

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Coppertone Water Babies 50 lotion took third place.

Those brands, which guarded against UVB rays before and after 80 minutes underwater and were "very good" against UVA rays, all cost $1.67 or less per ounce.

More-expensive products, including All Terrain AquaSport SPF 30 at $4.33 an ounce and Badger Unscented SPF 34 at $5.52 an ounce, didn't thwart UVB rays, the survey found.

What's more, All Terrain, which earned top marks in last year's survey, got a "fair" rating as far as protection from UVA rays, which penetrate deeper than UVB, though both types can cause skin cancer.

The priciest sunscreen, California Baby SPF 30+, at $6.90 per ounce, only got a "good" rating for UVA protection and a "poor" rating for staining fabrics.

The report speculated that the rankings may have changed because of new Food and Drug Administration labeling requirements for over-the counter sunscreens. The requirements, which went into effect last summer, mandated sunscreens be tested and labeled as "broad spectrum" - meaning sunscreens should offer protection against UVB and UVA rays.

The new requirements may have caused sunscreen makers to re-jigger their formulas to meet the new guidelines - although, as the magazine noted, neither All Terrain nor several other manufacturers changed their formulations over the last year.

All of the top-rated sunscreens in the survey offered broad-spectrum protection.

Karen Rauen, the director of health and consumer sciences operations at Consumer Reports, stressed the importance of wearing sun block.

"If you can't find one of the recommended sunscreens, your best option is to buy one that says it's water resistant, claims broad spectrum and has a claimed SPF of at least 40," she told ABC News. "And remember to reapply every two hours and after swimming or sweating."