Do not make high investments in links

[SEO]

Do not make high investments in links it is as putting all your eggs in one basket, what I meant with high investment is paying high dollar value or spending long time for a single link.

I see some people paying a lot of money for a link on a high PR website, other people killing weeks of their time along with a huge amount of money to get one link which could get you a good ranking but your ranking will be the slave of this link.
Links are not as gold, there is no guarantees that they will maintain their value for the next day (unless you are buying from very high authority website which regularly do not even sell links), any link on the web is exposed to many type of changes which could end it up with no value, here is some of them:

1- The website may lost its PR next update, or its PR might be edited by Google like what happened to www. w3school .com or www. phpbb .com which is the worst PR edit ever happened as they went from 10 to 5 .. imagine what happened to the guys those bough the links in the footer if they paid tens of thousands for a long term commitment.
2- The website may go off any time, may be the owner lost interest so he shuts down the website or sell it so somebody that has different policy or will create totally different content.
3- The website may adopt the nofollow policy anytime under the pressure of some expert's advice to stop selling links that pass PR juice in order to avoid Google penalty.
4- The website my stop updating/adding content so people will stop linking/deleting its links, think about a celebrity website, sport website or stock market news website that stops updating its content on regular basis, you will see a daily loss in links, even in if there is no loss, when your inbound links stop flowing your PR/link power will be affected eventually as the value of link juice fades by time since you have daily leakage in your inbound links for the same reasons (websites closing, websites adding nofollw, websites losing its value and penalized by Google).

So did that say I should go after quantity, cheap links?
I would see avoid even investing any money in direct purchase of links, you can use this money to build content or some legitimate link bait where you will get the better value of your money
BUT
If for some reason you need to buy links, mind the quality/relevancy of the link but do not spend too much money/time on one link.

[The Leader]

You gotta get your links from a large variety of sources so if google slams one type of source you will still have other types covered!