Your 404 is lame (your404islame.com)

Only one of the example sites seemed to have custom 404s. Unless I'm doing it "wrong", which I'm more than willing to accept may be the case?

yayyyyy!- http://yonenlabs.com/imaginarypage

booo - http://scarfilms.com/imaginarypage (this is just a standard not found error!)

booo - http://www.wryandginger.com/imaginarypage

booo - http://tether.com/imaginarypage

BUT - I like the idea, I like your designs, and I think an obvious, flat and low fee is great!

Their own 404 page: http://your404islame.com/non-existing-page

It is ok to put something funny on the 404 page and the pages for the other errors. But a proper and useful 404 error page will not only entertain lost visitors, but guide them back to existing pages of a website.

A sitemap and possibly a search input field are a good idea.