Paying for another child's place on a school trip

[Foxesandsquirrels]

My DD has just received a letter for an end of year trip. To not be too outing let's say it's to a trampoline park and costs £60. Her best friend will 100% not be allowed to go as it's too expensive and will need to choose the cheaper option, of which there are a couple. The girls really, really want to go to this trampoline park and I was going to take them in the summer holidays, but being able to go with their year will be far more fun.
I am happy to pay for DDs friends space, but I have no idea how I'd go about that. I feel like I'd put her in a really awkward position, and if I contact the school they'll probably feel like they need to subsidize her space. It's all around a bit awk. Do I just leave it, get DD to go to one of the cheaper options and take them in the summer holidays? It would be a really big burden off me if I didn't have to take them. This 'trampoline park' is far and a really big day out.
She's over at ours quite a lot so I can easily broach the subject with her, but again, I'm so conflicted. Do I just leave it?

I know this. It's the same at our school. It's the whole reason for my post.

I would just surprise both girls by saying you are going to pay for them both to go on the £60 trip together as you were planning to treat them in the holidays for working so hard in Y10 and you’d rather they go with school than you having to drive them there are back. I hope they have a great day!

Why didn't I think of this! I think I'll do that. That way DDs friend has the Sunday to get the permission slip signed. I'm sure her parents won't care once they know they don't need to pay. I need to do it this weekend as this trip option fills up almost immediately and it's whoever pays first gets a space.