I think Clive is right in that most hire yards will charge out at around 140 ppl – I know that Swancraft certainly do.
I think the simple reason is that the yards make a small profit which is built into their business model for hire charges. As Peter mentioned it is swings and roundabouts – if they sold the fuel cheaper or at cost they would have to recoup it in the hire charges so it all evens out in the end.
The likes of Goodchilds, Brooms, Horning Marina Services etc mainly sell to private owners and therefore are in a more competitive market but in any event probably make their profits from servicing etc so its probably a good way of attracting custom.
However to put it in perspective – I used 90 litres over 2 weeks holiday in the summer. At Goodchilds prices I would have saved 27p per litre or in total just under 25 quid. I don’t think I’d want to bother trundling all the way over to Burgh Castle to save 25 quid! It would be a different story of course if I had a twin engined sea going boat which would cost hundreds to fill up!