unthoughtful or underestimate?

So there's a friend of mine. She invited me to come with her to Singapore, next year. Sure enough, I refused politely. I mean, it's not that cheap, and I'm saving for my marriage next year. Then she said, "It's on me. We'll go there to celebrate my birthday."
Okaaaaay....
She continued and said that the airfare still fits her wallet, it's ok.
Well, then, Miss, I'm coming with you. Hahaha.
Besides me, she invited another friend who is close to her. This friend, way different than me, refused quite hardly. It turned out that this friend is concerned about her. Must be financial stuff, I assumed.
But she is hurt and dejected. She feels that nobody really understands her. She only wants to have fun with us, her friends. 
Ok, it's quite unpopular, celebrating your birthday with your friend by treating them going abroad?
But in other hand, she only wants to have fun. She has provided the price and believe me, she has her saving.
I don't know, whether I'm being unthoughtful by agree to her invitation so quickly? Or our friend, who is underestimating her by refusing her kind invitation?