Morning all...
I have an old set of Sony wireless headphones. The batteries are not holding charge now. I have taken the cover off and it has a Sony BP-HP550-11 rechargeable battery. Looking to replace, but Sony version is £23.00. Found other versions, with a varied price range. The power is 2.4v, 700 mah. it is two batteries. They are listed as suitable for various headphones etc, but as I said, loads of prices. If the battery is a replacement for the Sony one, surely they must all fit, regardless of price ?
Input appreciated, before I waste money. 
Not an easy one, no one wants to pay 23 quid for what, in essence, is a couple of 1.2v NiMi Cells stuck together inside a plastic case. Problem your going to have, as with anything like that, is eBay and Amazon will be knocking out Chinese copies for probably less than half that price, with unknown results, reliability and potentially safety issues.
Sony have been around a long time, there not generally a company for ripping people off, so maybe theres a reason why there charging that sort of money for thier own replacement batteries.
If your really keen, and handy with a soldering etc, you could break the battery pack apart and see if you could sort your own replacement Cells from Varta, CPC / Farnell or Maplins etc.
Other way of looking at it, how long is a genuine Battery going to last vs your enjoyment of the Headphone?, 5 years maybe, that works out at less than £5 per year or 40p per month, at least you can take the battery out of the box, plug it in and away you go.