Are you going somewhere, like a high tea for it, is that what the $50 is for? In that case I don't consider that 'being charged' as I would expect to pay for my attendance and I'd also take a gift. The gift for a kitchen tea doesn't have to be big - kitchen things, nice tea towels, a baking tray, some cooking utensils kind of thing.
Unless the invite says no gifts, take a gift.