Odroid H4+ ($140 for the SBC, $10 for a case; extra cost for memories) is an example of a cheap passively cooled version. It can also use a fan, but the fan is optional and not required.

There is an even cheaper version Odroid H4, at $100, which has a single 2.5 Gb/s port instead of two and it lacks SATA ports for HDDs (the fewer peripheral controllers also lead to a lower idle power consumption).