They'll more than likely be in the same price range as the headphones (earphones/earpods) and also the charging cables. So my guess is roughly in the £25 to £35 range for a single strap which is a lot but... that's Apple.
Seems like everyone is guessing in the $50 range. This is reassuring to me because I was worries that they would be upwards of $100. Hopefully they won't be but you never know.