For how long you want to put a link life time or for few months? I think 100-120 USD for a month is good.
Depend on the site quality too, if you are talking about PR4 link with 80 over external link, then it might only cost you $1~$2 per month.
It depends on many associated factors like Website themes, Domain Authority ,Domain Age, Page Rank of the Domain , Number of Outbound Links on pages . But still approximately ,it can't be more than $250/month if the site is excellent with the above said parameters.
Don't forget to follow this guidelines http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=66736
If the domain has millions of backlinks, I'll spend much more for a PR4 link from that site than if the domain only has 200 backlinks.