That budget is pretty tough for a good pair of entry level monitors.
You might be able to get a pair of Focal Alpha 50 for $450 if you shop around on ebay.
A pair of Yamaha HS5 would be around $400, perhaps cheaper on ebay, but I'd much rather get the Focals. They are a class over the Yamahas.
The most common recommendation at that price point would be the Rokit 5, but I would stay away from those.
Stay away from cheap consumer stuff like "computer" speakers, 2.1 stuff, etc.