The best return on investment is probably to add random newlines and multiple spaces, since most screen scrapers work from the HTML as text rather than as a XML (since most pages don't parse as valid XML).
The browser ignores whitespace, so your user's don't notice that
Price : 1
Price : 2
Price\n:\n3
are different. (this comes from my experience scraping government sites with AWK).
Next step is adding tags around random elements to mess up the DOM.
