Last time I transferred funds from my UK bank to my French bank, the charge was something like £20 I think. It's not a percentage (at least not at my bank), but it's enough that you want to transfer a large sum all at once rather than making smaller transfers.

There is also usually a charge by the receiving bank - I think in the US it's about $25. In France it's 18 or 20 €.
Cheers,
Bev