Sports jerseys can cost you an arm and a leg if you pay full price. This is especially true of authentic jerseys and throwbacks. These are fancier ones that cost more and are often the best looking ones around. You do not have to pay that much money though, as you can get a quality jersey for much cheaper if you know where to look.
The first place to look is to check with your friends and family and see if they have any great jerseys just laying around the house that are not worn anymore. Many times kids outgrow them and they end up just collecting dust. This can be a great way to get one for free.
I also like to check the local salvation army stores and see if you can find a used jersey there. They may not be in the best shape, but they will wash up nice and you will only pay a dollar or two for these types of clothes.
It will not be the highest quality of material, but if you are looking to save money this could be the best option.
Another place I check is the local paper for ads about garage sales. These are often all summer long and can feature some of the best bargains of all. Often times people will sell all of there clothes at once and you can get a bunch of sports apparel for dirt cheap. I have hit the jackpot many times using this method.
Good luck in whatever choice you use to find a good deal.