It's no secret that inexpensive products often work as well as (or even better than) pricey ones. But using a low-priced sunscreen may actually be better for your skin.
To understand why, imagine that you're hungry, and you have two jars of jam in front of you. One's your regular old Smuckers variety that you picked up for $2.39. The other is a $12 organic jam made with raspberries, peaches, and champagne. If you're like me, you're likely to smear the Smuckers on your bread with abandon — since, after all, buying another jar isn't expensive — but you might try to make the fancy jam last longer, since it's more costly to replace.
The same psychology works for skin care products. If you buy a pricey sunblock, you may use less of the lotion in an attempt to get your money's worth. But because sunblock needs to be applied generously (and reapplied during the day), you may get less coverage than you need. By buying an inexpensive drugstore sunblock instead of a more expensive one, you're more likely to use enough of it — and therefore, you're also more likely to be adequately protected.