I actually took a closer look like this, after someone on the WotC forums claimed there was enough Errata for it to be published as its own supplement. I'm not sure how relevant it is to the actual topic, but my findings were... somewhat interesting, so I thought I'd share.
The PHB clocks in at 320 pages, and is the product with the most Errata. 23 pages worth. However, keep in mind that the Errata looks somewhat more substantial than it actually is - it includes explanations of why changes were made, along with redundant information (as it posts both the Errata and, typically, reprints the original feat/power/etc in its new form.)
Trim down the PHB Errata to just the changes themselves, and we've got 12 pages worth. Remove the elements that are just fixing typos or adding minor clarifications, and we're probably under 10 pages.
The D&D Updates in total comes to 123 pages. Assuming a similar ratio of changes vs redundant material and explanations, we're probably looking at 60-70 or so actual pages of Errata. And again, if you take out typos and the like, I'd guess maybe less than 50 pages of genuine changes to the rules.
Now, 50 pages is a decent amount. But keep in mind - this is the Errata for 30 different D&D books, as well as 30 issues of the Dragon and Dungeon online magazines. All of that combined is something like 10,000 total pages of product.
So, how much Errata is there? Might be a lot, might be a little, depending on how one looks at it. I suppose everyone will still make a judgement call for themselves, but I certainly found the actual numbers on the topic somewhat interesting.