This is not a question of being "rational", and there's no right or wrong answer. People make trade-offs between quality and quantity of life every day. Do I want to quit smoking, drinking, drugs, and take up sports, etc for a chance at a longer, healthier life? Or do I grab all the fun I can, and if it costs me ten years, oh well? The right decision will look different for every person - and it's not black or white, but a continuum.

The choice of how much pain is worth it for how much of a chance at a longer life is going to look different depending on your religion, your responsibilities, your age, your experiences with pain, your family, your culture, etc. And yes, of course the cost of treatment will factor into that decision for both the doctors and the patient.