I think you were fine. Generous, even. Because either you have a birthday party, and your kid gets a gift and you pay for the entirety of the outing, OR you let the other kid come with you just as a regular outing and have the kid pay for his own activities and your kid doesn't get a gift.
I get that you asked, but I think since the other mom stressed that it was for a birthday, she should have declined the need for your son to bring anything. Because $30-$50 is NOT just spending money. That's a pretty huge wad of cash to fork over for a last minute invite for your kid to go out of town.
OP, when the mom was listing the activities, did she do it in such a way that it seemed like she expected you to ask how much money to send with your son?
(In a way, it kind of reminds me of a 'birthday party' that my nephew went to once. My sister was wondering about a present, and my nephew said that everyone just brings $X amount of cash for the gift. But then he had to pack up his XBox to take that along too; several kids were bringing their gaming systems and they were going to hook them together or somethng. I remember asking, "so you not only have to bring a present, but you have to supply the entertainment too?"