WebmasterWorld has a nice thread going about the value of different types of links. The thread starts with the question "what's a link worth to you in $?". It's an excellent question, and the answers are varied.
Although it appeared to be originally focused on the price you would pay to buy a link, a surprising number of people came back and pushed back on the notion, saying that it was better to focus on developing natural links. Pageoneresults has the following comment:
I look at it from the opposite direction. How much am I willing to pay for something that my visitors are going to appreciate, utilize and possibly link to?
For me, natural links are the ones I'm after. The paid links are a moving target, here today, gone tomorrow. Usually short lived and high maintenance.
Clearly he is focused on the longer term development of his web properties. This is an important distinction to make in your web promotion strategy. If you are in a competitive space, the high value authoritative links are the ones you want and need. Chances are that you can't purchase these with advertising dollars, and even if you can, they will be too high profile to carry any value for long.
But pursuing natural links still costs money. The time and money you spend developing high quality content, and developing relationships with the sites that you are targeting for in bound links is also an expense. Would you spend 10 hours of your time over a period of 4 months to get a PR6 link from a PR8 site?
It's a great question to ponder as you think about where your link strategy is going to be focused.