how much does it cost to buy a link? and where can i buy one? how much traffic did you get?

from the link you bought and how many links did you buy? did you get ten visitors to your website from one link, a hundred?

Links come in all sizes, there are no standard result figures. The popularity of the page providing the link is the biggest traffic generating factor, the somewhat outdated Google Page Rank factor is still a useful rough gauge of the popularity of a page, mainly indicating the number of other sites linking to that page. Some junk sites made just to provide backlinks may generate no traffic, getting a link from an influential site like the Huffington Post would bring hundreds or thousands of visitors eventually.

Buying ad space is a more direct way to get traffic, paying only for user actions (clicks) is one common pricing scheme, there are also vague paid traffic sources that typically provide low quality traffic or even software robot visitors unlikely to make a purchase, you can but 1000 visitor of questionable value for $5 at fiverr.com .

Buying links is most commonly done to boost the search engine ranking of your site, in the hopes of getting more 'free' traffic in the long run.

To paraphrase Google: Any links created for the purposes of search rank manipulation are in violation of Google guidelines and will be discredited.
Earlier this year a number of well known Private Blog Networks pages were removed by Google from their search results, effectively nullifying the effect of these paid links.
The NoFollow tag was devised by the search engines to indicate a link should not be used for search ranking, primarily an anti spam measure for webmasters to optionally use, any legitimate paid links like advertisements are expected by the search engines to have the NoFolow tag.

You can still buy links from webmasters, ideally through private arrangements that don't leave an obvious footprint, the recently added Google Penguin filter was expressly made to identify patterns of SEO Spam, that and Google's general renewed interest in site quality has made it more difficult to buy search ranking, a practice more common several years ago.