Both SMLS and Topping offer some small entry-level DACs around the $100 mark that will have an optical input and unbalanced (RCA) outputs.
You'll need a USB power brick for the SMLS as it uses that input for power as well.
Here's a Topping for $150 (the cheaper ones with optical inputs seem to be combo headphone amps):
This one comes with a remote and dedicated DC in with cable.
