To fly for a -121 carrier like SkyWest (or ExpressJet or Delta) you need your ATP.
To fly for a -135 carrier (less than 19 seats...all turboprops as far as I know) like Great Lakes, you only need a Commercial License. For those first year FOs with only a Commercial license, Great Lakes pays $21.22/hour, with a 75 hour guarantee: Less than $1600/month, before taxes. Initial Training Pay may be lower.