Short answer: Yes, you can buy it, no, it likely won't.
Long answer: It *is* cheaper for a reason, alx. A wrong battery (or a faulty one) can just as well short-circuit the laptop. It might. Chances are it will serve you a year or two, but are you willing to risk it? It likely won't have the parts, nor the care the Dell one would, if it's not just a packaged rock-brick (which I would suspect for 20 bucks).
It's just like buying non-brand printing cartridges/laser casettes. Many function with it for a long time, but they seriously damage the printer, some leak, some flat out don't work at all, many don't have the right colors and if you turn the printer in for warranty repair and the mechanic realizes you were using non-original material, your warranty is void.
Anyway, as far as battery goes, I'd be really wary of anything that promises to save you more than a third of cost of the original one, but in doubt whether to buy a 100 USD or 150 USD battery, I'd much more readily reach out for the original.