Prices vary wildly, fitment also differs, I have messed around with wrong fitment and moving the seat hinges or packing longer pins out with washers can get other tanks to fit, in my case that has been with cb550k3 tank ( flat filler cap) and cb500/4 tank with the chrome upstanding filler. The other issue I had was the cb500/4 tank has 2 outlets on the tap, easy enough to rectify with a m6 bolt rammed up the spare fuel hose and tightened up with a jublee clip. The change was apparently to get around the preceived risk to ones wedding tackle from an upstanding filler if braking hard, hence the flush lines of the newer tanks, also the bonus of being lockable. Personally I prefer the earlyer type cap with the chrome cap, but in this day and age the ability to lock the cap is something not to be overlooked. Keep yor eyes peeled at scrapyards and car boot sales, a few years back I picked up a cb750 tank with my eye on the chrome trim for £15 for the whole tank in a breakers, neither me or the breaker had much idea what bike it was off, an ebay search for similar ones revealed what it was most likely off so on ebay it went at 99p start and went silly expensive, to the point where I hand delivered it to be sure the seller was happy, which he was thankfully.
