comparable worth of backlinks and can 1 backlink make a difference?

I'm just wondering if there's any kind of comparable 'number' of what backlinks are worth?

For example if I could get 1 link on a PR7 page but had to put in a lot of work to write an article that would be human approved would I get the same result by just auto submitting 30 spun articles to PR 1 sites?

The reason I ask is because I wonder if it's worth spending the time targetting a small number of high PR sites for my backlinks or if I should just forget about that and go for a large number of backlinks no matter what the PR is?

Basically you want to do both. You want a diverse backlink profile with some solid links from high PR, trusted sites, and then a whole bunch of lower quality and lower PR backlinks. Preferably you want mostly d0follow and with your chosen anchor text, but at least as far as d0follows are concerned, I would always take a link with non-optimal anchor text over no link at all.

From my rather shaky memory, in some of the original Page Rank papers, PR was a logarithmic scale in base 8 (not a linear scale) meaning that you would need a shit load more PR 1 links. For example if you needed 8 links with an arbitrary value of 1 to attain a PR 1 on the toolbar, you would need about 2 million links of the same value to get to PR 7 on the toolbar. So a few high PR links can definitely cause some serious changes to your rankings.

Also bear in mind that there are probably other vectors of link value that pass through hyperlinks that are separate from raw PR, eg. possibly "trust" or authority, social or freshness value, the value of keyword anchor text etc. For example a bunch of retweets from Twitter might have technically low PR, but could be valuable in terms of being links from a trusted domain and also having a high social value. So there is more to link building than just mercilessly pursuing toobar PR.

I believe in natural quality links building, which google also recommend and you must follow this, if you want to win over google traffic

I've had 1 high PR link put me at the top before. It all depends on competition. I would recommend targeting the high PR links because they don't go away as fast as blogmeatspam.