Most collector websites report that a Princes Diana Beanie Baby in mint condition can be worth up to $400, but a quick search of eBay (http://www.ebay.com) reveals that Princess Diana Beanies listed, starting at $.99, have no current bidders. Remember, a collectible like this is only worth what someone will pay for it.
The actual 1997 Princess Diana Beanie Baby is worth about $100,000.00 if you search on ebay. Although a lot of variations on prices are being put on this particular item. You can just simply search for it over the internet and find how much it really costs.