ugh, do I feel you on the costs. I have an interview I was sitting on the fence about - then decided to go for it - in 24 hours the cost of a plane ticket jumped $130.
*@#%
I really don't think any program would invite you if they do not have a real interest in you. I tried to match last year and assumed some programs just invited me out of curiosity or something- I know I ruined my chances by being somewhat suspicious of invites, and lacking inherent confidence. This year I think I am doing better just by simply realizing they want me.
But these programs are busy and run by professionals. They get many more applications than they grant invites, even poorly competitive programs. They do not have to invite anyone. If they invited you they think you are a good choice.
I posted this somewhere else but there is a movie called HITCH with Will Smith. He is an expert who helps guys with no game hook up with girls they like. In one scene he is talking to a guy right before a date and says something like : She is going out with you. This is good.
That means she likes you. At this point your job is NOT to try and get her to like you, she already likes something about you. Your job is simply NOT TO BLOW IT!
I think the same thing is true here. Nobody in a residency has the time , money or inclination to invite a bunch of people just to laugh at them behind their backs. They usually pay for hotels - they are not going to waste that money on someone they are just going to ignore. If they asked you on this date they like you and are really interested in you.
Life is getting prettier and I'm starting to see colors. 
Just be patient about hearing back.
