How much have you/did you spend on interviewing for radonc? It seems like the magic number of ranked spots is 8, so I figure 8 interviews + 3/4 TY/prelim years is a total of about 12 interviews? Am I in the ballpark? Did you guys end up taking out extra loans to cover the cost?
The magical numbers may be ~8, but if you get 12 or 15 offers you would be doing yourself a big disservice to turn them down to save money.
To answer your question, it depends on where you are going, how long you will stay, where you stay, whether you rent a car, etc.
For me, the average interview went something like this:
$200-300 airfare
$60 cheap hotel on hotels.com or the like, 1 night
$40 car rental, 1 day no insurance
$30 food, gas, misc., etc
= about $400 per interview
Sound about right? This may be different if you can manage to fly to several places consecutively in a single trip (though if you have to stay longer to do so you will of course spend more on hotel, car, food, etc.), and hopefully several places will be close enough to drive, though depending on where you live, this may be affected by winter weather. From where I was in Milwaukee, I was too chicken to drive anywhere beyond Chicago.