Has anyone been in a online relationship where the person wanted to pay for their plane ticket to come and see them?

No, but the moment they ask for money I'd block and move on. I never send money to anyone.

When I was on EP, there was a user who befriended me and then kept messaging me about her "relative" that had "cancer" and needed money for the "medicines".
I let her beg for a while, then I blocked her lol