How do you determine how much to spend on a truck?

I figure out what I want to pay and go from there not what I can afford. I set a limit of 35k for my last truck and wasn't expecting to be able to buy anything new but came across a deal on the one in signature. My tow vehicle is an extra that doesn't get much use other than towing, my last 06 truck had 44k miles when I traded last Sept. Spending 50k + is something I have a hard time getting my head around, it's not because I can't though.