That's not how it works though.
Deciding whether or not to provide a drug is based upon how much you get out of it and how much it costs. Most of these cancer drugs are complementary therapies added to existing routine treatments. You are looking at incremental gains, but basing decisions on how much better someone is and how much extra you need to spend to do it.
Quality of life and "life expectancy" are combined as quality adjusted life years. If you live one extra year at half the normal quality of life, you get 0.5 quality adjusted life years. Deciding on providing that treatment comes down to what it costs. $50,000-$100,000 are the usual ranges you see for one QALY. If the acceptable cost is $50,000, the drug can cost your $25,000 and be considered a good use of health care money. If it costs $200,000, it is not a good use of money.