Just be aware when buying from petsmart you risk them needing vet treatment, which would cost extra. They could be fine, but most have worms.
If you do go to get the petsmart ones--no rescues. Get the healthiest ones you can find--clear eyes, no stuck sheds, thick tails, fat bellies.
Make sure you have a bowl of pure calcium ready--no D3 in the bowl. Pure calcium only. A little baby will need plenty. Still dust their food with calcium+D3 and vitamins often.
Keep the enclosure CLEAN! Worms are shed in their feces, and being around feces means that the worms keep building in their system. If you get one that is wild caught, keeping the enclosure clean as you can will help prevent the worms from taking over, and a healthy leo is better able to fight off these kinds of problems.
If they still refuse to gain weight despite proper conditions, they may need dewormers at the vet. You won't need to bring the leo in, just bring in a fresh fecal sample. Usually somewhere between 15-30 dollars. Then you will know if there are worms you need to treat. But you will need to have it measured by weight, which may mean bringing the leopard gecko to the vet anyway.
A leo from petsmart could be just 30 dollars...or about 100 or so when you add all the vet bills.
BUT not all breeders are made equal. Just because your LRS breeds them doesn't necessarily mean they are healthy.
Go for the healthiest possible to get the best deal, basically. Vet bills are when the cost starts to build up.