It's all relative.
I have a site-wide link on a PR7 site and it has helped and contributed in increasing the authority score of my web site tenth-fold over the past 12 months that the link has been in place.
Common sense says that Google is not going to class all these links the same and most will not even be scored or weighted.
But you have to do your background checks on the web site you are getting the link from. Check who they are currently linking out too, how much are they linking out to 'sponsors' and even things such as there navigation structure etc.
Check
www.archive.org for age, check Yahoo! or MSN for backlinks pointing to the site you wish to advertise on, even go as far as to check out the background of the other sites that also link to the one you want to advertise on.
It's whatever works best for you. I can not drum it in how much you must research first. If you are buying a link, think long term. Don't expect to get that much trust sent to your site for a good 6 to 12 months, if not longer.