It is $1.5 billion baseline for each repair mission. Using the car analogy, at some point the monthly repair is more than payments on a new one.
Well a $1.5 billion service call is a lot of money, but how much does a new one cost? I still suspect we are using a flat tire as an excuse to buy a new one, when it makes sense to fix the flat tire and build a betterone. Then give the working Hubble to the kids, when our better version is ready.