In the US (I'm not sure about in other countries), medical spending is skewed heavily towards people who are already very sick and often dying, while SENS is focused on delaying the onset of that state. Even if work on prolonging lives is over-funded, the funding that goes into it is likely being directed primarily to much less efficient ways of doing it.
Existing expenditure probably isn't effectively directed. However, SENS doesn't seem to be very much better. It's oriented towards biomedical gerontology. It seems pretty obvious that the way to produce potentially long-lived minds is to create them in a digital substrate - so that they can be copied and backed up. SENS seems to be pretty irrelevant to that project.
Givewell’s Holden Karnofsky, who has previously posted his thoughts on Givewell supporting SI/MIRI recently discussed the potential for Givewell to begin evaluating biomedical charities, in Givewell’s Yahoo Group. Someone suggested (as I have through less direct means) that they take a hard look at SENS Research Foundation, and then Aubrey de Grey appeared and began an interesting discussion with Holden.
The thread begins with Holden’s long initial post about Givewell’s stance on investigating and recommending biomedical charities, which is definitely worth the read for greater insight. The rest of the conversation is aggregated below for anyone else who can’t stomach Yahoo Groups’ interface.
Overall, Holden seems to agree with the goal of SENS, and interested in the details, but the conversation seems to have ended in October 2012 with Holden stating that he was waiting for Dario Amodei’s thoughts on SENS.