$30 USD per link would be reasonable for good quality links, depending on how you qualify them, for instance, on relavent pages with less than dozens of links and all from different IP addresses.
By posting this I am not advertising that I will pay this much for links, so please don't contact me wanting to be a link hound for $30 a link. Nonetheless, $30 is a reasonable price for enduring quality links.
There isn't any simple formula. No two people will agree on what is a "good" link or a "relevant" link. If you need lots of links, you should look at hiring someone offshore but get to know them for some time before setting up an ongoing contract.