If you can't afford to pay for someone at your wedding, you don't invite them. Asking them to pay for your wedding is (IMHO) out if order and just bloody rude.
Especially if only some of the people were paying! If I discovered that I was paying to be a guest at a wedding while others were not, I would be furious!
Also, wedding invites are not tit for tat! Just because you are invited to theirs, doors not mean you are obliged to invite them to yours!
If you are that desperate for them to attend, you would have had them on the guest list from the start and budgeted accordingly for them.