If they are wanting you on retainer at all times, I would go for a monthly salary, with per diem for overnights. I don't know what a good monthly rate is for this type of flying, but it should be something worth it to you. If you just do the daily rate, and they slow their flying, then you are hosed for the month(s) they do not fly. If you do decide to do daily rates, though, then make sure it is in your contract for all days to be paid equally (i.e., no 1/2 pay for your "vacation" in Mexico).
Make sure you get whatever your agreement is in writing. DO NOT go on a hand shake and verbal agreement. You will lose in the end. For per diem, average pay is $100/ day while on the road and $125+ for international flights. Negotiate what you can with them, but that seems to be the norm.
Make sure your contract has in it what level of hotels you are expecting to stay in...usually 4 or 5 stars is acceptable. If you know the operation and know they are staying in Motel 6's then of course don't ask for 4-5 star hotels.
I'm sure others will chime in. Good luck and let us know how it goes. Hopefully, this thread doesn't end up like some others on here...