I don't know if $20 million per year for these two rovers is too much or not, but on the whole it's not a lot of money. Of course at some point they will have to shut them off, because even though they have survived for much longer than originally expected, they will not last forever. Also, NASA will send another rover to Mars in a few years (as will ESA). But it's not a lot of money for every tax payer, to do peaceful science and enhance our understanding of the fascinating planet Mars. A lot more money has been spent on much more dubious projects.