I think your benchmark will be the TT Pro license and the CQG Trader license rates. They are proven, and are about $1800 per month.
Didn't ET poster Rosy2 put an autospreader, including source code, up on github or sourceforge for free? IIRC, it wasn't a full-featured autospreader, but the basic functionality was there.
Ah, I see that link, but I only see jar files. It also for IB only as far as I can tell. I don't have an IB account to mess with it so I can't comment. The one that I would build would be far more general.