Welcome to the Forum.
I agree with
@Merab's Slave that does sound about right.
The thing I would ask first is why they need to keep her overnight?
The x-ray is critical in checking for stones, but not sure why an overnight stay is required?
Did the vet explain the reason behind that?
I have had various piggies x-rayed in the past, and they only ever required a quick whiff of gas and were back to normal within minutes.
They may want a urine sample, but that is something you can easily collect at home.
So firstly I would clarify exactly what they plan to do, and if they have expereince with guinea pigs.