There's two ways to add a price on.
1) Default cost. All adoptables, no matter what, will have this as the price (unless you set an individual price for the adoptable *pokes number 2*)
2) Set a cost for each individual adoptable species. Each species gets it's own price.
It's not a shop, it's right on the adopt.php page but likewise you can set 'cash and promocode' to get a purchasable adoptable for cash, which is how many people make shops on MA.