See the real key issue here is why is it low on freon in the first place.More then likely you'd refill it and it simply just run low again real quick and then you'd end up waisting the freon refill kit for nothing.
I'd go and look around , call around find a A/C shop that offers a low cost A/C system check and then have them fix the leak and refill it for you.That would vary in price but if right shop you might only end up spending 100 bucks instead well over 150 to 200 bucks.
Hope that helps and best of luck.By the way make sure if you do buy one of those kits buy the higher grade kits bec the seals on those hose valves end up going and leaking if you try to reuse it again next yr etc on those cheap kits.Always make sure you know the max amount of A/C pressure you can fill the system to and never try to add the freon to the high pressure side always the low side, plus some of these cans have a special additive that is blue in colour etc and if you see it showing up on the A/C lines etc that means it's leaking at that spot or close by it.
