TENS means transcutaneous electrical nerve stimulation. You place electrodes on the skin and they produce impulses which have a pleasant sensation and help the body produce it's own natural pain killers. Don' t know prices bur smaller simpler models are less expensive. I think Boots do some models and might be a good place to start to find out costs.
Go to your GP and ask if you can be referred to a physiotherapist first. They will try and help with your pain, and that will include lending you a TENS machine. Don't buy one, as it might be no good!