The United flight search engine makes searching for mile upgrades relatively easy; after you've gotten past the initial search, you can go to advanced search options and search for mile upgrades. It'll show you the fares for Economy, PP, Business, and the cost in miles to upgrade with Economy or PP to Business. It'll also show if the upgrade will instantly clear or if you'll be wait listed. Of course, the sticking point is actually finding flights where your upgrade will clear.