Well it's also unfair if we assume large difference in the marginal utility of wealth -- for example to go to extremes, we might assume that a $10 contribution

Well it's also unfair if we assume large difference in the marginal utility of wealth -- for example to go to extremes, we might assume that a $10 contribution from a low-income individuals represents the same sacrifice as a $1,000,000 contribution from a high-income individual. If that were the case, a $100 contribution from a low-income individual represents 10x the utility of a $1,000,000 contribution from a high-income individual. So in that case the lead would pipes have both more contributors, and higher utility per contributor, than the art. So total utility would be maximized by giving more money to the lead pipes.