I am sure this must be a common issue, but for some reason I can't find a way to track it down via web searches. 

Here are the basics:

I'm divorced, male, mid-40s. I am the noncustodial parent of a 17-year-old daughter. 

Just as background, I have been willing to split all relevant travel and most other expenses with my ex, in keeping with our separation agreement. But I also have agreed to other things, such as cat-sitting, for the sake of our daughter. She is very attached to the cat and seems happy that he stays with someone he knows when she and my ex are both away. I make less than my ex does, but I speak up about expenses only when I feel I am being taken advantage of.

My ex has applied for and received a travel grant for a trip overseas, which as I understand it, is an award. As long as she documents expenses, I believe they reimburse her for them. She contacted me by SMS to say she wants to take our daughter and has asked me to split the child's airfare with her. Here, I am limited to my memory from a previous grant that she received, but the essence is that they will "cover" her airfare and hotel, etc., as long as she can document it. 

That's fair enough. But when I told her if this is her trip, and it is a grant, she should be able to pay for everything from the money she gets back, she texted me this: 

"If you're not able or willing to split these non-covered expenses, I may have to reconsider taking her." 

This bothered me for two reasons: 

1. I have never been "unwilling" to try to help my daughter travel or otherwise get something that my ex wants for her (usually travel or school expenses). I want to tell my ex to stop using that kind of language when discussing these issues with me, but I don't know if this is a battle not worth fighting.
2. A ticket to Africa is kind of expensive, so it is tough for me to contribute. And I can eke it out if necessary. But I resent the insinuation that my ex will blame this on me if I don't contribute. Especially if she gets enough money back to cover the whole ticket for this particular trip that she initiated on her own. This is not something my daughter has to do for school or anything. I feel this is a veiled threat that my ex will pin this on me and say, well your father didn't want to pay for you go with me on the great Africa trip.

At the same time, I don't want to respond in a way that escalates this or gives in to her pushing my buttons. I asked her to switch this discussion to email before I could agree to anything. So at least I can think this over a bit. 

What is the best way you think to handle this kind of situation? 

**Edited to update**: Thanks for everyone's answers, including those people just getting to this question now. I just wanted to add a prefatory note that I have had some time to look back on this issue. Basically, I have learned a little more about how to pick my battles, and my daughter went on the trip, I contributed money, and she got a lot out of the journey. My particular battle picking involved deciding that for me, a certain amount of money was worth the peace of mind of not having to dispute with my ex over the financial baggage. I can totally understand how other people would feel differently. But I can see the light at the end of the tunnel with child support, and after that part is over, I will deal as much as possible directly with my daughter and leave my ex out of the financial equation. I was mixing up that money part with the reality that when a divorce happens with a child involved, there will always be at least some communication necessary with the ex, especially if the child is not yet an independent adult. I'm dealing with that as its own issue and trying to keep it separate from any legitimate financial concerns I might have about splitting costs with my ex. I hope this thread is still useful to others, but if the moderators feel it can be closed, I am OK with that, too.