Upgrade DAC for an Oppo BDP103

Greeting friends. I recently acquired an Oppo BDP103 as an all in one solution for my audio/video needs. My primary use is for audio playback of cds and a few SACDs plus the occasional movie. I've been toying with the idea of adding a stand alone dac to improve the 2 channel audio reproduction. I've not dipped my toes into the water in this area before so I'm curious, how far up the dac food chain do I have to go to realize an audible sound improvement? In a dollar a cents basis, $400.00, $500.00 ? Used of course would be a strong consideration. Any recommendations for a reasonably priced dac that is not a lateral change? I appreciate inputs and commentary.