It would vary depending on the brand you get. I used to buy the contacts that cost me about $20 a month. Of course, the optical store may have a discount sometimes. If you see a deal like that then show it to your parents and ask them to let you try them.
Go to your doctor.They will tell you how much you will spend according to your vision problems. If you have an astigmatism, then they will cost you more.If you just wear regular contact lenses, then they will be much cheap.