The only costs obviously are travel, hotel, and ERAS.
Really the best way to save money is to schedule your interviews regionally. January is a crazy month for some reason, so you can get two interviews in on a weekend easily. For example, I know Columbia always interviews on a Sunday, and there are other NYC/NJ programs that interview that weekend. So stay on top of your invites and try to schedule another that weekend. And if you can't get the date you want, schedule a firm date and ask that you be put on a waitlist for the date you want. People cancel all the time as the season goes on, so you might end up with a better day.
Boss posted a list of interview dates here:
http://www.orthogate.com/option,com_for ... ,2541.html
Drive to as many interviews as possible (although with gas the way it is maybe that doesn't hold true this year...). If you schedule early and can take the time to drive, you'll usually save $. Sometimes the offers come with only a 2 week notice, so flights can be tricky.
Orbitz had good fares when I was interviewing. The downside for any travel is that the weather isn't great in December and January, so flights can be delayed (especially on the East coast).
On the other side, if you are a few interviews back to back you may miss some of the night before dinners. I dont think this factors in to your admission, but you can find out some good info on those dinners.
Hang up your suit immediately after interviews, so you can save a few bucks on dry cleaning.
When you are scheduling, talk to the secretary about other interviewees wanting to share a hotel room. Or post on orthogate that you are looking to share a room. You'll meet new people and halve your cost. Or if you have a former student from your school at a program, you might be able to crash with them. This is controllable cost, and can take off a hundred or two per interview, depending on where you are staying. Share cabs if you can at places where that is convenient.
Some people asked if they could interview during their away rotations. I had mixed feelings about this, because usually the programs rank the applicants after the interviews, so I think you will be better remembered if you interview on their set dates. But you would save on a trip if you interview during your away.
As far as ERAS is concerned, apply to as many as you feel necessary. Don't let that cost sway you.
The bottom line is that overall a few grand is peanuts compared to matching in Ortho. So even if it costs you $5,000 to interview at all the places you want, if you match you are gold. If you skip interviews or don't apply due to cost and don't match, you'll be kicking yourself while you are miserable doing something you don't really want to do.
Good luck guys. Ortho is a sweet field, in extraordinarily high demand. There is nothing better than getting that awesome letter on match day, knowing that you'll be doing ortho for the rest of your life. Plus chicks dig orthopods. Something about them being keen on bones...