I think this is a narrowsighted view. First of all, you're thinking in very small numbers, but for the sake of simplicity...
If a website costs you £100 and 100 people view it, you can see it as meaning it was £1 per person, except that you're not paying a developer to bring you visitors, you're paying him to make the product (which includes, in part of it's design, natural mechanisms for building traffic).
Also, as the months go by, you'll get repeat users and new users (if it's updated) so that £100 you spent dilutes. So in the first month, if you get 100 users, that's £1 per user. In the next month, if you get another 100 users, that dilutes to £0.5 per user, then £0.25 the next month, etc. etc.
When you're paying for marketing services such as SEO or banner ads or something like that, that's when these kind of ROI (Return On Investment) statistics come in handy.
When you're attempting to figure out the worth of a site if you're having it made, you value it by looking at market prices for the creation of other sites with the same features. If someone else paid less than you did for a site that's noticeably better, then you can induce that you over-invested in that site.