Well, I just slightly remember some figures concerning Eisenia foetida earthworms. Its quantity (real case scenario) can raise 3-4x times within about 6 month. Temperature range 10ºС-25ºС, humidity - not less than 50%, optimal acidity - pH 7.0.
Ground beds should be separated (at leat 1.5 m) and have a little inclination for the water drainage.
$700-1500 - equipment cost
$30-45 - price per 1 kg of worms