I spent $12.00 on a NC Star, and worked fine with my old M4. Though in some certain situations red dots are useless, you just look for where the bbs are going and you're usually set. No need to spend more than $50 for a red dot if you're going to engage in closer combat. Though the more expensive holographic sights are enjoyable to have, yet pricey, around $100-$120. If sniping, a rifle scope with magnification when centered, "zeroed in" indeed makes the role, and ability to land your bbs much easier. I would not spend more than $70. Luminosity during the day is a key factor too, nothing like a faint red dot, when a fresh battery is used. Some plexiglass, and a hallow drill bit usually works if you can find some laying around if you want extra protection, file around the edge to make it fit.
