Do you mean the number of opportunities in the future, or the ability to donate larger amounts of money right now? We could do:
What is the most dollars you would be willing to donate in order to save the life of a randomly chosen human? Assume this is the only opportunity you’ll ever get to save a life by donating—all other money you have must be spent on yourself and your family.
and also the endowment effect reversal:
If offered a choice between saving a random stranger’s life and an amount of money, what is the smallest number of dollars you would have to be offered to choose that option? Assume you will only be offered this once, and do not have any opportunities to spend or donate money to save other people.
Your version seems good too, though I would worry that introducing the temporal element and background of progress might bias things in some way.
It seems like that question would interact weirdly with expectations of future income: as a college student I donate ~1% of expenses, but if I could only save one life, right now, I would probably try to take out a large, high interest loan to donate a large sum. That depends on availability of loans, risk aversion, expectations of future income, etc. much more than it does on my moral values.