I went on about 20 interviews and this is my personal opinion. Everybody travels a bit differently.
Car: Depending on which city I was in, I tried to see if public transportation available (NYC, Boston...). If public transportation was not feasible, I used priceline name your price to rent a car. I believe I got a bid in Houston for around $15/day before taxes. Sometimes I bid on an economy car, got rejected, but offered a mid-size for the same price. I drove to all interviews that were about 10 hours from me, so I had a car for most of my interviews.
Flight: I used a combination of Southwest and Kayak to book my flights. The nice thing about Southwest is you can check a bag without fees. Kayak can help you find cheap flight combinations such as flying one way on one airline and flying back on another. You can sign up for individual airline reward programs, but keep in mind that the "points" usually expire. Southwest also does not charge if you have a change in plans. I did get caught one time and had to reschedule an interview because all flights into NYC were cancelled due to weather. I had to reschedule the interview till the next day (the program was very nice in accommodating), offered to get bumped off my original rescheduled flight for 4 free roundtrip tickets and still arrived in NYC the same time the original flight landed. The only thing about losing that day was I prepaid for a hotel, which I lost a night.
Hotels: I stayed with friends and family when I traveled if they were available. If not, I used priceline name your price to bid on hotel rooms. You don't get "rewards" such as free nights, but I would say that I saved about 33% on average off the list price. There is also another tab where you don't have to bid, kinda like a "secret hotel" in a certain area. I used that also. Also, remember how out you bid if you don't have a car and once you prepay, you're locked in. Another way you could save money is "Airbnb" where you stay with people who rent out there apartments. Have to be careful, but a very good way to save a lot of money, especially in NYC.
For the 20 interviews, I spent approximately $2500. My first trip to NYC was the most expensive.
I hope this helps and good luck!