Moved to V+ forum (as I assume you're asking about watching it on Virgin).
BBC HD itself is free.
But to watch it on Virgin, you need a V+ box.
This costs £150 (new customers) / £100 (existing customers) for installation, with a £5 monthly fee on TV "Size M" & "Size L" (there is no monthly fee for V+ on TV "Size XL").
http://allyours.virginmedia.com/html...lus/index.html
http://allyours.virginmedia.com/html...rs/v_plus.html
On Digital Satellite, BBC HD is Free-To-Air (FTA), & requires an HD STB (e.g. Sky HD, e.g. Freesat HD).