When do you throw in the towel?

Sometimes it's a crapshoot... We always buy used diesels for farm/ranch work and sometimes they can go on for years with very little repairs and sometimes it's one thing after another. We buy these trucks knowing that there will be repairs along the way but the key is we buy diesel trucks that don't have EGR's and DPF's. Having said that we had one truck that everyone hated to drive because of the manual transmission we cut our losses after the transfer case took a dump so off she went after a list of repairs. Early this year we had another truck purple aftermarket transmission took a dump and the dealer didn't want to take it for the core. It's currently on the short list to be sent on it's way.