What is the best (affordable) soprano sax brand?

You won't find a good new soprano under $700, so you're going to have to go used. Good sax brands include the "big four" (Selmer, Yamaha, Keilwerth, Yanagisawa) as well as some other brands like Buffet and Stephanhouser. With used instruments it's impossible to say what you'll find, since it depends on the age, condition, model, and how much the seller wants for it. Craigslist can be a good starting point for looking, and music stores will have sopranos on occasion too. My best advice is to look around, find a few in your price range, then play test them and check them all out for condition, intonation, ease of playing, durability. Then you make the ultimate decision on which one is right for you and your wallet.