Hi there -- I'm Dr. B from Texas. I have been a small animal (dogs & cats only) veterinarian for over 11 years, and working in veterinary medicine for 18 years. All of my experience has been cutting-edge, high quality practice where specialty hospitals with advanced imaging (MRI/CT) were just minutes down the road....
I'm so sorry Sam is going through this! Back pain is not fun for anyone. The toughest part is the unpredictability of it, and the absolute necessity for strict confinement, which is very hard to do as a pet owner (speaking not just from veterinary but also personal experience).
It sounds as if your vet has at least diagnosed Sam with a back problem. It's tough to really see these on plain radiographs, and often advanced imaging (CT or MRI) is necessary to see the true problem and its extent. That being said, the only reason you pursue advanced imaging is if you have the full intention of immediately following through with back surgery. Sam would be put under general anesthesia for an MRI, then if a surgical lesion is found, the doctors would immediately transfer him into surgery. All in all, you would be looking at anywhere from $3000-$7000 for imaging plus surgery (depending on the cost of living/cost of veterinary care in your area). I don't ever recommend an MRI unless the pet owner is intending and able to follow through with surgery. Otherwise all the money, and anesthetic risk to Sam, is simply a "gee whiz" scenario.
So when do I recommend MRI and surgery? When the back problem is causing NEUROLOGIC problems in the legs. Many, many times, the swelling of inflammation around a slipped/bulging disk is causing all the pain. As the inflammation/swelling goes down WITH STRICT CONFINEMENT of the pet, all the pain resolves and the pet is able to resume normal activity. Typically this takes 2-4 weeks. However, if Sam is experiencing neurologic symptoms (not just pain) -- unable to properly place feet right side up when standing, dragging feet, straight/rigid back legs -- then, yes, that means the disk itself is either bulging so much or has completely ruptured into the spinal cord. This will NOT resolve with rest and pain meds. This is a surgical emergency. Those clients of mine who cannot afford surgery at this point need to realize that the only likely option for their pet to be mobile again will be to purchase a dog cart (wheels that attach to the back end of the body to allow mobility).
So if Sam is just too painful to get up into the car/couch/bed, but he still has normal use of his legs (can stand normally, walk), then rest and confinement without moving forward with an MRI is very reasonable. Obviously, if his condition worsens, then you reconsider MRI + surgery.
Here is a link that can give you a summary of this condition: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=3223
Now, there are other back problems that can cause a dog to have difficulty walking (fracture causing pressure on the spinal cord, spinal stenosis/narrowing of the spinal column, infection in the spine/vertebrae, cancer in the spine or spinal column, to name a few). Although you mentioned the "recent or old" injury to a vertebrae, that is not very common. Disk disease (bulging disk) is VERY common. With a fracture, you can often see the malalignment of the spinal column due to the fracture, which would result in pressure on the spinal cord. If your vet can see definite malalignment, AND there are neurological signs in the legs, then yes, MRI and surgery are indicated. Otherwise, if no neuro signs are present, and the other diseases I mentioned above aren't seen on radiographs, it is very reasonable to take a conservative approach with STRICT REST and anti-inflammatory pain meds (Rimadyl/carprofen, Previcox, Metacam, etc -- NOT Tylenol or ibuprofen!). The robaxin is the least important treatment for disk disease.
I hope this helps you. Please let me know if you have any other questions. Otherwise, be sure to accept my answer so I know you got the information you were looking for. Good luck!