2000 Jeep Heater control box

I am located in the Chicagoland area and I have a 2000 Jeep Grand Cherokee. Since last winter, the heater has been off and on. Now it only blows cold air. I was told it needs a new heater control box. What should I expect to pay for this?

Is it worthwhile to look for a used part since the vehicle is 9 years old and has 140k miles on it?

Any help is appreciated.

I assume that the control for the box is controlled by vacuum air and that has been checked first before saying the box is bad.

I had it checked at a local Pepboys which I usually find reliable. Is there anything else that would cause the heater only to blow cold air?

Do a web search on Heater Treater - it’s a DIY fix for heater box issues. About $100.