I took him over there this morning and he was admitted. It turns out that the $5000 figure my vet gave me included the MRI, surgery, other testing, etc. So I'm really glad that has been cleared up. The MRI itself is only $1500 (BIG difference). He's going to have an ultrasound today though, because they also found an abdominal mass when they examined him (in the area of his prostate). Once they determine that it is benign, they will do the MRI for his spine tomorrow. They think it is a disc problem but they said it could also be cancer of his vertebrae. So we still have a lot to learn, but at least we're on the road to finding out everything that is wrong with him.
Thanks everyone. It was good to hear about other dogs that have had MRI's for much less than $5000. It put me in a more receptive frame of mind.