Most will be free. I have switched to EDF who will supply an economy 10 meter free of charge and do not charge a leaving fee. So after this is installed I plan to return to Scottish Power who currently have the cheapest Economy 7 rate and are willing to supply it to an existing economy 10 meter (but can’t install it themselves). This will give an extra 3 hours of off-peak usage on the economy 7 tariff and I believe will be the current cheapest option. But it might be best to check what type of heaters you have and if they can be configured to come on at economy 10 times, otherwise maybe economy 7 is best for your needs.
[ed: Scottish Power are no longer accepting E10 customers, but there are a lot who will – check out the list of Supplier here: http://economy10.com/list-of-suppliers/ and the discussion about Suppliers here on the forum: http://forum.economy10.com/forumdisplay.php?fid=6%5D