To give less for a wedding as the couple haven't paid for it themselves and I've not got a +1

[Itsawd]

Usually I'd give 50 per person for a full day thing.

Seeing as the couple I'm going to at the weekend have had it all paid for by parents I'm thinking about giving less just because other couples I know have had to save up for their own wedding. Aibu?

No.
I give a standard 50.

You clearly don't want to go.
Politely refuse the invite, send a congratulations card ( without money) and do something you want to do instead.

WHat i do when I get invited to a wedding is carefully work out how much my "place" costs ( making a deduction if the Bride and Groom have us a financial contribution from their parents. Then I give a gift of the equivalent value minus £10. This way I ensure I make a profit every time I attend a wedding.
Sadly Christenings and Birthday parties tend to be less lucrative so I only go to those if I REALLY like the people involved

Tight as a crab's hole.

I've never considered how much the wedding has cost when I give a gift.

The same as children's partys Ive never given a bigger present if the party has cost more per head.

I give what I can and give more if I am closer to the bride and groom.

I do however sympathise with the op about the plus one. It must be pretty hard to attend a wedding alone , especially if everyone else is coupled up.