August is an expensive time to be traveling in Europe, and traveling on a Saturday makes matters even worse, so I don't think you'll be able to get a non-stop flight for less than what you have found, at least not without being more flexible with dates. You might consider flying Vueling from Lisbon to Madrid, and then connecting (on Vueling, Easyjet, Ryanair, or something else) to Paris, but that still wouldn't save too much if you can't be flexible with dates at all. Skyscanner.net is a useful site for finding fares on this sort of route.