Heater core needs replacement after thermostat replaced?

What is your question?

Hi, My 2003 mini cooper is currently at a repair shop. Originally, I was told that only my thermostat needed to be replaced and that it would only be $600. Then, as they were getting ready to give me my car they said that my heater core needs to be replaced (including all of the hoses). This will now cost me $1400. Is it really worth it? Did the mechanics do something to cause this? Do I need to replace this to have my car function? Would it be best to just get another car and trade it in? Thank you for your help!

How long has your 2003 Mini Cooper had this problem?

a couple days