(2 par post):
This first bit might seem a bit pedantic, but it is relevant. You state that you currently get all of the free channels. Actually you're almost certainly getting all of the 'free to air' channels, but not the 'free to view' channels.
'Free to air' channels are unencrypted, so you can receive them with any satellite digibox. 'Free to view' channels are encrypted, so they require a valid viewing card to watch them. Currently the only 'free to view' channels, on the Astra satellites which Sky uses, are the 'Five' group. (i.e. 'Five', 'Fiver', 'Five US' and their '+1' versions). I'd be extremely surprised if your Sky+ box currently gets those channels.
Up until recently, Sky were advertising their 'Freesat from Sky' service. Customers who didn't have a satellite dish could get one installed, together with a Sky box and a 'Freesat from Sky' card, for (originally) �150. At various times the offer price was cut to either �100 or �75. It's almost certainly where you've seen the �100 figure quoted in your post.