What 50k reward, who says they would have gotten it, who says thats what their plan was. If the McCann story somewhat inspired them, surely finding the kid limits what they can get, while if they had the same situation as the McCann's, a fund with FAR more than 50k in it so they can go around looking for their kid would be a far bigger draw to them wouldn't it.
If they killed the kid in the future it would, stop any possibility the kid will tell on them, and open up the possibility of a lot more than 50k, how do we know they were going for the 50k. If they find the kid, the chance of more than 50k goes out the window.
Even if they claimed they were going for the 50k, at this point, caught, its the least evil thing they could claim to have been doing.
Aswell as the whole thing, so they release the kid, where would the do it, how would they make sure they were the only ones there, how would they explain how they happened to be where the kid was released and how could they make sure they were the ones that found her. Drive her in the boot somewhere, ditch her, obviously have to get away from the car they brought her in and get to wherever it is in some other way so as not to seem too obvious. In other words, theres not a single way they could get that 50k at all.
The kids picture is all over the paper, say Donovan releases her at one end of a shopping centre, and Karen is on the other side, theres a couple thousand people that could "find" her and get the reward before Karen could. If they did it somewhere not busy with no one around, its ridiculously suspicious, the only thing that makes any sense to me, is they were going to kill her later on and go for the grieving parents sympathy give us some money thing, or if the body isn't found, play the McCann card and try to get sponsorship to "find" their kid, taking holidays around the world.