I agree w/ everyone else - no it should not be assumed or expected. I have a good one for you though..
in 2007 we (DH & I) took our two boys, as well as my younger 1/2 sister & brother to WDW for 8 days. so we had a 4 year old, 2 7 year olds and 1 8 year old. We paid 100% of the cost for my brother (7) & sister (8) as well as the 4 of us (me, dh, ds1 (7) and ds2 (4)). me & DSis & DBro share the same father, and they have the same mother) anyway my father & stepmonster, opps, I meant step mother

did not send $.01 towards the trip even for spending money. nothing, nada, zilch. We asked them 1 year before the trip if we could take the kids with us when we went. said we'll pay the transportation, tickets, food, hotel. we told them they would only need to send some $ for souvies. they agreed & we planned the trip. a few weeks before we left, dh & I took DS & DB to get new clothes for the trip (everything!) and when we took them back to thier parents house, my stepmother actually started telling me some of the suggestions of souvies to get the kids... & to make sure DSis got this and DBro got that..etc.. I asked her what was thier budget for souvie money and was told, well, we don't have anything to send with them.. (WHAT!?!? you know a year in advance that your 2 young kids are being taken to WDW and your ONLY expense is some $ for souvies and yet, you haven't managed to put aside ANYTHING for them??? We did notice that DF & DSM had plenty of $ throughout the year for ciggies & beer though) ugh... DH & I were so annoyed. Of course, we would not buy our children something in front of dsis & dbro w/o getting for them as well... so we found out at the last moment that our souvie budget was doubling... Don't get me wrong, DH & I don't regret it for a second (DH is a saint amoung men let me tell you!) We know its the only way those 2 will be able to go somewhere like WDW before they are adults.
(Wow.. well that ended up being longer & more Off Topic than I intended!)Well I guess my point is, I would not expect a divorced parent to pay for the other parent's leisure trip. yes, they should have to share the costs of school trips or something like that, but just your typical family vacation.. umm.. no.. not even spending money...I commend you for the sacrafice you make to take your child on any trip you can, however, don't hold it against the other parent for not contributing, thats not fair to the other parent. you have to take what "you" would do out of the equation. you can't be disappointed in someone for not doing what "you" would do, because, they are not you.