If your doctor refers you to a dietician due to a medical necessity as in you need to lose the weight or it may be a life or death situation then the health insurance might cover the cost of the dietician.
Most health insurance though won't cover the cost of seeing a dietician so it's kind of hit and miss as to whether the health insurance company will cover the cost for you to go to a dietician.
Seeing a dietician out of pocket costs around $50.00 to $150.00 per visit to the dietician so it's not that expensive although still not cheap.