the cost will depend on your area... In my area most opthamalogists cost about $130+... but in other places I have lived they only cost about $50, especially at walmart or "mall" places like lenscrafters. Call a few places and ask them! of course glasses are on top of that cost and can range from $30-$40 on up to really expensive designer frames.
If you are really strapped for money... ask the school nurse if there is any state programs that could help you get exams or glasses for your son until you can get him on insurance.
Don't make him wait till summer. When I was a kid I "passed" my eye exams the first few years by cheating on them! I was terrified of the "consequences" of failing, so my mom didn't have any idea that I had poor eyesight until I was in third grade. Even then, it was only when she noticed that I wouldn't cross the road (because I couldn't judge how close cars were) that she thought I might have an eyesight problem. When I was tested at the eyedoctor, my vision was terrible! Teachers and nurses had never noticed.
After I got glassed my reading level went from "struggling" to college level within one year... spelling scores shot way up, and my teachers who had suggested putting me in "special" classes for the key areas of reading and writing, moved me into the "gifted program". Problems that seemed like learning issues had actually been bad eyesight! Waiting to give your child glasses could really put him at a disadvantage, because he could not be able to see everything written on the white-board at school... and doing "ok" isn't good enough if your child could be excelling!
Check and see if you can get financial "help" from your state or city with the glasses- if you cannot afford about $100-$200 total out of pocket expense right now. some things in life can wait... but eyesight isn't something worth waiting for unless you absolutely have to!
-M.