Very image oriented site. Hugely varying quotes. Average?

Hello, I have a site at www.picklesadventures.co.uk which was created before mobile sites were really a 'thing'. All my other sites were basic blog types and cost next to nothing to created mobile versions of. I've had quotes for this ranging from £50 to £750.

Am I being scammed at such high quotes? Or are the lower quotes completely underestimating what needs to be done?