Well, if you know it's right, then you pay: who's stopping you?
And I'll take care of what's right for me on my own, and will count my money myself too. I'll definitely prefer to spend it on cancer research. After all, why should I worry about AIDS?
It's incomparably less likely that I ever get AIDS than that I get cancer. And it is so not only for me! It is so for most people. How can I get AIDS? I don't screw men in the ass; I use protection in general -- and at any rate, hetero sex, even if unprotected, is a very unlikely source of AIDS, at least in the woman-to-man direction; I don't stick used needles in my veins; I don't need regular blood transfusions (which are rather safe these days, so even if I get hit by a car and need one, it's not gonna be a likely source of danger.) Now cancer is another story: statistics are very clear on that -- and unlike AIDS, getting or not getting cancer is largely out of my control.
Where should I spend my money? If you say it's right to spend it on AIDS, is it wrong to spend it on cancer research? There's only so much money that can be spent at all, it's a zero-sum game here.
Just asking, mind you.