About how much would glasses cause if...?

I'm looking to buy glasses on my own without telling my mom because I lost mine and she might not buy me new ones 'cause she'll get mad again -.- I just need a general idea of how much money to save up before I leave to buy a new pair of spectacles. If I get one with a grade of 200/20 both eyes (nearsighted), how much would it generally cost? Also I'm more likely to buy medium-thick rimmed glasses, of plastic or any kind of light material. How much approximately would this cost? (And do prices drastically differ somehow in other countries? 'Cause I'm not going to buy these with dollars :D I'll just use a converter or something.)