Gosh...what a situation to be in...first you have to decide if you are going to go or not...
Husbands sometimes figure out if they ignore a situation it will just go away (it is a mysterious phenomenon). Then when it doesn't go away, suddenly make a quick decision about the cheapest way to "fix" it. There I "fixed" it now be happy.
You know your husband best how will he react to you saying you and your son are not going? Will it drive a huge wedge in your relationship? Do you think this get together will the the first and only one for years or will this be something they do every year and expect you guys to attend?
If you decide to go...do NOT drive...6 days in the car for your son...and 6 days for you to be angry and mad at your husband it way way too much. Just pay the extra money and fly...$400 more (even that you don't have is way worth it over a car trip of that magnitude).
Don't forget about the other costs when you get to AZ...rental car, hotel, eating out, trips to local sites...etc etc. Or your husband offering to take everyone out to do something (eat, amusement activity, etc)...so factor that in.
I am never one to say just to do something you don't have the money to do...I like to save up and plan. But it all depends of how much these means to your husband and how long it will affect your relationship, and if you will be able to put your feelings behind you and have fun and be cordial to everyone on the trip.
You say you feel caught in the middle...that is exactly how your husband feels...caught between you and his family...he doesn't want to talk about it...remember the mysterious way if he ignores it, it will go away or be fixed...he knows you can't afford to go, but he feels like to his family that he can't "afford" not to go.
I say maybe tentatively plan the trip...how much it will cost (flying, put your foot down on driving) and other expenses. Then tell him you have figured out how much it will cost then ask him if he is okay with the total and if he is okay with paying off the amount over the next 6 months. If he says yes...well, go and try and have fun...AZ is beautiful. If he is not then let him know how much it will cost for him to go alone..and tell him to have fun.
Oh, and call your SIL and give her your email address and ask to be put in the loop next time as you are the "family planner and scheduler". if my husband's family tried to figure stuff out with him I would find out about it 6 months after it happened or when I saw the pics on facebook...