5% isn't so bad. Although, with a character builder, any player books are pretty much obsolete...
We're still below 10% after essentials.
The key is to remember that the majority of the errata is about 2-3X longer than it needs to be, and adjust the page-count there.There are 23 pages of errata to the PHB 1.
The PHB 320 pages.
That's 8% even. Assuming that the document is no more than twice the length it needs to be (because it states the errata and then reprints the power block/paragraph(s) that it effects), we get 4% change to the PHB 1.
Note that there is much less than one full page changed for every full page taken up by the errata. I would hazard a guess that there's been less than a total of 1% of changes to the actual rules and powers.