There are a ton of reproduction carts around, especially recently, but they're not really particularly expensive to manufacture as mentioned above. But they're usually marked as such in the places they're available like Etsy and eBay. At best I would recommend staying away from any deceptively cheap eBay copies (think like $20 or lower, for your particular game), or any copies that look "too" perfect condition. One of the better things to look for when seeing if a cartridge is a reproduction or an original is to check the label itself. Reproduction carts almost always have overly glossy labels, so they're super easy to spot from pictures. You can also look at all the edges on the cart (like on the Nintendo logo on the back, or the two strips on the front of the cart), real carts have a much "sharper" line, whereas repros are softer and less defined because of the casting process used.
I actually personally don't mind getting modern reproduction carts (that are purposely sold and labeled as such, of course), because modern manufacturing practices have a bit more quality than sub-2000 Chinese fakes produced years ago. I've got some SNES repro carts of some of the more expensive games (like Earthbound and Chrono Trigger) that cost $15 each and play identical to the original games with no issues so far.
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Skelletonike:
