2010 Great Wall V200 heating issue

I booked my 2010 Great Wall in with my dealer to look at an overheating issue. I also asked for a service to be done at the same time, but the dealer completed the service before looking into and fixing my overheating problem. It also appears that I have a cracked head. I asked the service department what is the worst-case scenario, and was given a price of about $5000. The issue I have is my 2010 model isn't worth very much, so if the price tag to fix it was high I just wouldn't bother fixing it, I'd just go buy another car. The other issue is they are saying that I'm stuck with the service fee and that it's my fault that they serviced the car first and didn't fix the possible major issue. What advice can you give me?