MekHQ 0.3.18 version.
With and without ATB on the stats window the cost is always -1 cbills for every unit. This comes up in the purchase window and the unit market window. Screenshot attached.
I checked in MegaMek and it isn't coming up in the add window screen so I'm thinking it is a MekHQ issue. I did not check MML, though.
This bug has been found and fixed for the next release.
the fix was on the MM side so I have no commit report.