I need to make a one way flight between two cities in Europe. The flight I want to take on a European airline in 500 Euros for a one way ticket. If I buy it as a Round Trip ticket, it is only 127 Euros. I plan to buy the Round Trip ticket and then cancel the return leg. Does that seem reasonable?
If you call and cancel your return leg, it gives the airline the opportunity to re-sell your unused seat to a standby passenger. Seems better than just not showing up. Also, I've heard - but I don't know if it's true - that if you call and cancel, you can hold the unused portion for credit (with a change fee) towards another flight.
Don't even bother cancelling the return leg, just don't show up. I've done this before, e.g. once had to fly from DC to Boston and RT was $50 less than one way.. As long as you take the first leg, no one cares.
There are numerous bargain airlines that service city-city within Europe. Ryan Air, Easy Jet and Vueling are three big names, but there are many others. All will gladly fly you one-way without any sort of penalty or shenanigans - and will probably do it for less than 127 Euros. Look into it before you go the big airline & skip a flight route.