Do you mean including connecting the two televisions to the Sky box? If so that’s the bit that’ll cost a lot. The actual hardware is £10 or so plus a similar amount for a second remote.
Personally I used the co-axial sockets that were already in place in each room – went in the loft, bypassed the old analogue aerial and joined the relevant cables for living room and bedroom. Works perfectly, far less faff than having to load the app each time. Picture’s more than acceptable on the 21″ TV in the bedroom.
Connected to the RF2 port in the back of the Sky box, which I think all should have, including the newest ones.