well, if it is a standard syphon and the cistern comes off the pan without having to cut it off, i would expect to pay about forty quid for the labour and a tenner for a new syphon, coupling washer etc.
of course, some bone-head may have glued the cistern to the wall with three tubes of mega-grip in which case, another hour (at thirty quid) is charged as you try to cut it away from the wall.
you may also need a new close-couple kit if the bolts and/or plate are rusted out.
and then there is a chance that the syphon is an odd shape to fit a narrow cistern. in which case, you may end up paying twenty quid for a particular brand of syphon.
if all goes well, the job can be done in twenty minutes.
O.K. then 90 quid!
pete