1993 Toyota Corolla Question: How much would it cost to repair the defroster for the windshield?

When the weather is cold and rainy here in Virginia, my windshield will get all foggy and streaky decreasing my visibility greatly. I've cleaned the inside, tried directing the cold/hot/or warm air toward the windshield, opened windows - everything to get it to clear up. The air flow doesn't seem to be very strong from the vents on the dash (which should blow directly onto the windshield). I'm on a small budget, but desperately need it fixed. What should I ask them to look for and about how much should it cost?