The cost of private medicine is variable. You need to shop around for the best value and experience.
The price of a private ENT consultation with clinic endoscopy for full examination, can vary from £250 to over £800. Experience is the key, being in Harley Street might be convenient for some, but that has no bearing on the quality or value you get. Doctor's CV's aren't scrutinised before they are allowed to practice there, no interview or vetting process occurs. It often attracts doctors who are simply about wanting the prestige of the name, so that they can charge more.
I know this because I spent 20 years there. Prices became so ridiculous I decided to move out and set up my own clinic in South Central London. It's called HealthHub. Easy access as it's under the station at Herne Hill. £250 for ENT consultation and endoscopy.