Why does it cost so much to advertise jobs online?

I am absolutely gobsmacked at rates charged to advertise jobs online. Why do they carry such a premium?

Here are some of the sites I’ve looked at advertising on (most expensive rates first)

• Guardian Jobs - £750 for 30 days
• TheBookseller.com
  • £315 for a month online ad inclusion in one email bulletin.
  • £250 for one-off small panel ad in their weekly magazine – interesting that print is cheaper than online in this case.
• Chinwag – £149 for 30 days
• Leed.co.uk – £146 for 30 days
• LinkedIn – £130 for 30 days
• PublishingTalk – £60 for 30 days
• Twitjobs – £49 for 30 days
• Gum Tree – £28.79 for 90 days – This was exceptionally good value. Very happy with the price, quality, and quantity of responses.
• Free listings
  • SYP Jobs board
  • BookCareers.com
  • IPG Jobs board

So far, Gum Tree has delivered the best results in terms of quantity (and probably quality too). Second would be LinkedIn.

Edit – Added a few more and corrected LinkedIn & Chinwag sort order