Cheapest reverb to stick in analog audio circuitry?
Another option is a Microchip Microstick. It comes with a 40 MIPS dsPIC chip that is suitable for implementing a reverb algorithm. Audio could be sampled with the on-chip ADC and output using PWM; very few external parts would be needed. The dsPIC chip on its own is about $5, it's a lot less in large quantities.