Antibiotics shouldn't cost too much from the vets. I think I was charged (not including the vet appointment) around £7. However, and this isn't me trying to scare you, longer-term treatment of sick piggies can be very expensive depending on their illness.
Completely unrelated to your pig's symptoms, but I have spent nearly £700.00 on Little Pig in the last 6 weeks or so (including x-rays, ultrasounds, swabs and other tests). I would strongly recommend starting a savings account for future vet bills or getting pet insurance.
I do not have first hand experience of this as didn't get my pig from p@h, but I understand that if you keep the vet bills, they should pay you back for it.
I do think you should take your pig to the vets first thing in the morning though, if not tonight (but emergency vets cost even more).
I'll keep my fingers crossed your pig is okay.