I would like this too.
As far as I can see the only way to do this would be to redesign the payment system completely so that the payment fields are in the CatLinks table rather than the Links table. In this way you could have different payments for different categories - e.g. expiring at different times - which is what I need. I doubt that this will happen in the near future (but I wish they'd done this from the start). At the moment I am having to add the same link multiple times which is not sensible. (I haven't upgraded to GL3 yet so not sure what implications this will have).
As far as I can see the only way to do this would be to redesign the payment system completely so that the payment fields are in the CatLinks table rather than the Links table. In this way you could have different payments for different categories - e.g. expiring at different times - which is what I need. I doubt that this will happen in the near future (but I wish they'd done this from the start). At the moment I am having to add the same link multiple times which is not sensible. (I haven't upgraded to GL3 yet so not sure what implications this will have).
