Some worms are wriggling a lot in this can I'm holding. Maybe I should open it.....

Why pay them? If you're going down the eugenics route - using whatever criteria to decide who can be a parent, then you may as well go the whole hog and not pay them.

(yes, I appreciate this is volountary)
You've got two sides to the argument - firstly, there is an encroachment on a persons health and what they might perceive as a human right to procreate (or even a human right to get jiggy without thinking of the consequences). One example the Daily Wail recently held up as a beacon was a couple who were actually deemed too stupid to be parents, so their kids got taken into care - despite what seemed to be relatively lucid interviews with them where they said they wanted to be, and could be, good parents. There's other folks the Daily Fail like to write about - for example the woman who keeps trying for twins, and currently has 13 kids.
On the other side of the fence, you have the "think of the children" crowd, who would bash those that are not responsible enough to be a parent - there are probably ample studies in the social services that demonstrate that such families might have a higher tendency towards abuse than most.
The main problem with applying this to addicts of a variety of types is that there is a potential for change - they need not remain addicted for life, and there is a potential to reform and become "better citizens" (assuming that their addiction interferes with that). I guess you could implement a reversible sterilisation technique, though. Maybe those that get sterilised have to pass a "citizenship 101" test to have the procedure reversed

?
You could apply it to "genetically unsuitable" folks, but then you have a raging debate that will Godwin itself (see
http://en.wikipedia.org/wiki/Godwin%27s_Law) very rapidly. There are those horrid, incurable genetic flaws (Huntingdon's, for example) that might be eradicated by offering incentives to carriers to not reproduce, but I can't see that offering an incentive would work in this case unless it is a fairly significant sum.
Personally, I might be in favour of it, but the controls on it would need to be so convoluted it might not be an effective technique - so you'd end up in dubious ethical territory quite quickly. For example, what's to say that the addict has sufficient mental capabilities to make that decision - or is (s)he just after £50 to spend on more substances to abuse his/her body with?