PR still matters.. anyone telling you that a PR0 link is worth the same as a PR7 link should change industry. You’re going to tell me “but it’s pagestrength that matters..”, which I will agree with and tell you that the PR7 page has more page strength than the PR0 one.
To answer the original poster’s question.. aside from “as low as possible”, personally I never pay and would never pay more than 5-10$ for a link like that UNLESS it’s a new page on a website that has high PR and is well established.