I bought one of these about five years ago from my local club who were selling off some donated guns that nobody ever shot; it certainly is not 10m. match quality but was in nice un-marked condition. Since there was no interest in it I paid £20 which went towards the club funds.
I would think that I actually paid less than its market value and would guess at £30-40 being a more realistic price for a private sale. It is really only a back garden 'plinker'.
I only bought it for novelty interest having never seen or shot an underlever air pistol before; I don't think many underlever variant pistols were ever produced. I only identified what I had bought from Dennis Hiller's book where it is described as "The Center" and I have no idea what the Falcon is.
I hope this helps
PS - I have just spotted a mention of El Gamo 'Falcon' in "The Blue Book of Airguns" with no illustration and only a brief description as an underlever cocking pistol with a 7" barrel; the 'Center' has a 14" barrel.