A bit of wet finger to the wind but I suspect between £120 and £200 depending on condition and where you buy from. You may get lucky at an auction where no one else is really interested (was that a pig flying?). Other people may differ in prices but I think I am in the correct ballpark. My prices are for a rifle 'you expect to work as it should' with price adjustment being for largely cosmetics.
I added this latter part because I was once caught out buying a boxed 124 that looked excellent. There was no facility to examine it more than outer looks, although the cocking cycle felt smooth. However, a home test quickly showed problems in the power plant area and a strip showed a crumbling piston seal. Now had that rifle been bought by a person not able to sort out the problem it could have been an expensive buy when you add the cost of a new seal and labour for fitting. If you can tackle such things yourself, fine but if not, it might be wise to look for a guaranteed fully working model. I hope I am not teaching granny to suck eggs!
Good luck.
Cheers, Phil