1999 GMC Savana air conditioner problem

I have a 1999 Gmc Savana Conversion Van. The problem is with the air conditioner. We are getting cold air, and I can hear the blower blowing, but very little air comming out. The fan seems to be running, you can hear it get louder when you change the blower from low to high, and occasionally it will work fine, but intermittently, and seems to be getting more frequent, you cannot feel much air comming out of the vents. The little bit of air comming out is cold. Someone told me it sounds like the vacuum seals gone bad. What do you think? Also, around how much is a fair price for this repair?