Investors should be willing to pay a premium for a quality stock, but that doesn't always mean you must pay a premium price.There's never a bad time to buy a good stock. A good stock is an even better buy, however, at a lower price.With that premise in mind against a backdrop of relatively steep valuations for most stocks, here's a closer look at three long-term prospects currently trading at bargain prices.1. DuolingoDuolingo (DUOL 6.03%) is one of those names that rings a bell, but you may not be exactly ...