OP, is there a reason why your MIL cannot watch your niece while your FIL drives you to the airport? If your flight is extremely early in the morning, then he should be back home before your BIL leaves for the stadium but if not, can't she handle it alone for a little while?
I'm sure she can but seems like they are not willing to make it work if FIL backed out of the favor already.
OP, I truly understand that you are just venting about the whole thing. I read the ENTIRE thread

and it sounds that it truly isn't just about the money but the entire situation. I'm sorry that you feel let down by family after you thought you could count on them since they already promised to do you this favor.
Sounds to me that you have your financial house in order and DO help family in need. It's not that you would throw them out on the street for not reciprocating "favors." It's just that it truly is HURTFUL when they not only take you for granted but seem to be extra unappreciative of you.
Sometimes it is (or should be) better to vent to strangers. If it has already been decided that you and your husband aren't going to let his family live on the street, then venting to him about them can cause unnecessary friction between the 2 of you. It would be different if you truly can't afford to "put them up" and HAD to give them tough love. Of course, you may be enabling them - but that is a whole other thread! LOL.
Sounds to me that the financial burden of providing housing for them isn't breaking you but is just causing you to be even MORE frugal than you naturally are.
My parents and my family always take each other to the airport because the frugal side of us just can not FATHOM spending over $100 or parking or car service when it's just a quick drop off or pick up. HOWEVER, the closest airport is usually 15 minutes away and the farthest one we use is about 45 minutes away.
When we are talking about over 60 minutes away or 2 hours round trip (in your case 4 hours round trip) then the cost savings really isn't enough for us to make the drive. In our case, we would never book a flight at an airport so far away. We would pay more per ticket to fly from a closer airport. (Not everyone has a choice of a closer airport.) But if it did require such a long drive, we would just have to account for the parking.
Our time is money and 4 hours of driving (2 round trips) is worth more than $100. Might be different if it was $300 for parking.

8 hours of driving (2 4hr rd trips) is a lot of driving to save $100 IMHO. However, I also understand that different people put different price value on their time.
OP, I hope you find an affordable alternative to having your FIL drop you off. I hope after a while you can let go of some of the hurt and bitterness in your heart.
Maybe try to look at it from your FIL point of view in this instance that he is not rejecting YOU and your family by backing out on a promise to do a favor for another family member. Maybe he feels that this is a rare opportunity to see his other granddaughter that he never gets to see (you said SIL doesn't let them see her). I think if I were him, I might jump at that chance too and hope that you would understand and be able to find other means if I told you early enough.
I hope you all have a WONDERFUL trip!!
