Depends where you get it, when you get it, box or not and general quality. Also depends on your currency and country.
Here it's on sale used for $40 USD. I got mine on Kijiji for $20 CAD. You can get it on Ebay used for the $20-60 range, give or take.
Don't spend more than $60 for a copy. You can get it that price or cheaper. Look at your local options. Look at Facebook Marketplace or Buy and Sell apps for listings; compare your options. Be sure to note to yourself if you want new or used, box or not. All these affect price, but shouldn't gameplay. If you're okay with used copy without a box, check your local pawnshop every once in a while and you might see a copy in the $30 range (again, depends on currancy and quality)
Note if you buy online, the game has to be from the same area as your 3ds. A Japanese copy of UM won't work on an American 3ds.
Tl;dr you could probably get it as cheap as $20, but will probably have to settle for $40 range. Don't buy a copy over $60.