Well, I think you do have to go to her funeral if she dies. I mean, she is your mother in law. Not going would probably make you look (and feel) like a jerk. Still, I don't think it's right that you should have to pay for everything. I guess your current husband isn't the father of your son? Seems okay if he were to stay with his Dad (or whoever your ex is) while you're gone, as long as he's a good parent.
What I think would be fair, would be for your husband to pay for your flights and lodging (and everything else that you needed while there) and you pay for the stuff that you have to do here, like your horses etc. Or the other way around. There's got to be some way to manage it with him helping. I agree with
Spirit that he sounds a mite controlling if he wants you to pay for everything.
He's probably "telling" you to go, rather than asking, because he expects you to be there for him if his mother dies. That's sort of one of your big jobs as wife. Even if he acts like the impending death doesn't bother him much, it does. My husband's father died several years ago, and he was very businesslike about it, but let me tell you, it really killed him to see that man buried. He desperately needed me, even if he had a hard time saying so.