Interesting question and one a lot of small businesses will be curious about. From our research we reckon the average cost is about £70 per 1000 emails for a business list, and much lower for consumer lists.
This does vary enormously. If you go to ebay and search for email lists you will often find 500,000 for £99, or even less. However the majority of these email addresses will not work, or will be local authority, schools, MPs, charities etc…
There are some very large providers including www.experian.com who may be very interesting as they offer market segmentation.