I know it's frustrating to feel like BM has gotten some benefit while you see your husband continue to shell out for whatever his kids seem to want. (though the daughter's trip sounds like it is due to some special effort on her part.. I would hope he would be supporting that)

But.. the reality is your SO paid HIS share.. only of both the sports and trip costs.  The fact is that it was BM's dad that paid HER share of that trip.

And.. while the trip may have cost 1200 it's possible some of what her dad was paying for was so that his granddaughter would have some spending money for the trip too.

The reality is that you can be jealous that BM has a dad who is able to help pay these things for his grandkid.. but your SO doesn't have anyone to help subsidize his half.. it's unfair in the karmic sense but it unfortunately is the way of the world.. some people have better resources.

BUT.. he should not be complaining to you.. you know it.. and the fact that he can't stop dumping his problems on your head is an ongoing problem and theme for you.

For some reason our SOs can't help themselves and tell us all the stuff about their kids that we really don't want to know anymore, and that has caused battles and fights in the past. I think it's them trying to vent and wanting us to listen but have no opinion about. They don't want to hear what we have to say because it's a questioning of their Disney ways. But their are consumed in sadness and worry about their teenage spawns.

Last night we went out for dinner, and without me asking anything SO told me that SS19 (who has a car given by his BPs,who can barely hold a job and is barely hanging in there in community college, and has sh!t for btrains) bought an old 1970s beat up car non-working car and blew his savings. I only said cool what year, color, is the car and asked a few more questions STRICTLY about the car so my disengagement is not so glaring - I didn't ask anything related to SS19.

I know this kills SO inside that the irresponsible kid bought this junk that took all his savings and will need more $$ to repair. SO has been trying to teach financial responsibility to this arse (after many years of me trying to and being ignored or frowned upon). I did not engage in the spawn's actions - which recently I had and got burned becaise SO was complaining and fretting about the dumb dumb buying this stupid car, and I said that they as BPs are enabling the disaster becasue SS does not feel any financial burn in the two houses, that they need to charge him rent, or make him pay some house bills and his own cell phone so he can feel that money is hard to get and to teach him financial responsibility. SOs reaction was bad - like oh you again putting down my precious baby, and criticizing my parenting style, yadda yadda yadda.

Now, SO is fretting, dumbass bought the broken car to fix and more $$$ will be needed, and I am disengagedly just talking about the car...

This craziness doesn't stop at 18.  Or 20.  It's goes on there whold life. BM has her ex and her BF Paying for her. She living the life when you are doing with out.  That hurts. Someone should of told you that the money doesn't stop at CS. Payments.  
'You need to get on paper a family budget.  Including all expenses. Rent. Food, car, insurance, money going to fund a retirement account. Money going into a vacation and entertainment account.  New car account.  House repairs account.  CS. [ medical, account]. What ever money left over can go into SK sports and trip money.  SO may have to get a second job.

Just make sure you are not funding SK unneeded things as expenses traveling sports teams  part of getting divorced is the kids get to miss out on things.  

One of my broken record comments.

Unless otherwise COd, CS is the NCP's sole duty to support their failed family progeny.  The CP is to manage their own funds and the CS they receive from the NCP to cover all kid related costs including extracurricular activities.

We learned this during sports and band.  We attempted to recover half of equipment costs from the SpermClan. Nope, the Judge set us straight.

So, not one Cent more than CS unless otherwise stipulated in the CO.

Beyond CS our CO required the SpermClan to pay two other cost elements.  They owned the costs of visitation travel for them to get SS to their location for visitation.  We had the duty to pay to return him to our location following visitation.  They also owned paying for half of any medical costs not covered by Medical insurance.  We had the duty to pay the other half of any medical costs not covered by medical insurance.

Keep it stupid simple. Pay only what is COd keeping the onus on the CP to cover anything and everthing else Skid related in the CP household.

For the NCP, pay for gifts, care, feeding, travel, activities, etc... when the Skid is with you.  Provide not one Cent more to the CP beyond what is COd.  

Since BM in this situation has a sugar daddy BioFather of her own, she can snake money out of him for anything beyond what is COd.

If the skid drinks the PAS Kool-Aid when CP plays "Your dad is not paying for XYZLMNOP" sit the kid down with the support spreadsheet and show what is paid to the CP on a monthly basis highlighting the cumulative total at the bottom of the sheet.  

It works both ways. In our case, the NCP SpermClan claimed that the CS they paid was buying us new cars, new houses, fancy vacations, SS's nice clothing, etc, etc, etc.... while his 3 younger half sibs by two other baby mamas were starving and doing without. So, we reviewed the household income/budget spreadsheet with SS showing what little the pittance of $133/mo in CS truly was.

The kids eyes bugged out of his head during those post visitation clarity sessions when he would come home with the SpermGrandHag PASing crap fresh in his mind.  The next visitation when she would try it again and he would call her on it she would call my DW losing her ever loving Hag/Harpy mind.  "Why does he know all this?".  No recognition that she was the one who planted the lies in his brain that we were then were duty bound to counter with the facts.