This article is from the eBay FAQ, by William B. Shaw firstname.lastname@example.org with numerous contributions by others.
When eBay bids up to your maximum amount on your behalf, the bidding system
automatically determines the next bid based on a simple schedule of
increments located at http://pages.ebay.com/help/basics/e_item11.html based
on the current bid price. The higher the item price, the larger the
increment, bid increments range from a nickel to $100 on the U.S. Site.