$400-500/mo is a good estimate but it depends on what kinds of trips you fly and how often. Someone that flies day trips and 2-days is going to see significantly less per diem than someone who does 4-days. Personally, I wouldn't go putting it down on a mortgage application as income since it is meant to cover expenses on the road, which do pop up (van driver tips, food, transportation, hotel if you can't commute home, etc). I kind of look at it as a company-provided "rainy day fund".

And Christ guys! Tip your van drivers! A buck is NOT going to break the bank! If money is that tight for you, might be time to sell the boat, the sports car or the trophy wife. If I fly with one more captain who doesn't tip a van driver who is waiting at the curb, opens the door, hefts the bags AND offers you a coke when you get in the van then I am going to rip those 4 bars right off his or her damn shoulders! Ever wonder why van drivers seem less than thrilled to have to deal with airline crews? Its probably because most of the ones they deal with are miserable, whiny, demanding and cheap! A dollar and a "thank you" can go a long way to earning some goodwill at the bell desk.