I have just been asked 250€ for the instalation of a beolink and a pair of BS 3s!!!

...I meant BL3s and BL2s....sorry for the venting....

Depending on how long it will take to do your link and how much cable is used, would have a direct reflection on the price.  If, for example, it would take half a day with masterlink cable included, I would say that's not too costly.  If it is only an hours work, then clearly it is too much.

That's not at all ridiculous, as meandmyshadow says, although there isn't much kit, wiring MasterLink into a house can be a very costly thing to do, adding the price of labour and the cabling itself.