Tub boats currently cost 12¢/mo for 12 units of capacity, compared to wooden barges at 0.02¢/mo for 50 units. That's 1¢/mo/unit for tub boats versus 0.0004¢/mo/unit for wooden barges, or 2500x the cost/mo/unit.
edit: tub boats, not box boats
I forgot there was a bug reports section: this should be moved there.
The pak balancing spreadsheet (https://github.com/jamespetts/simutrans-pak128.britain/blob/master/Pak128%20Britain.xlsx) shows that tub boats were considered to have one driver per boat at Coach Driver rates. This must be a mistake: there is no room for any crew on a tub boat (http://www.tonycanalpics.co.uk/wmidlands/shropunion/shrop/04421.jpg). I speculate that the entry was misread as tug boats, which would be a super easy mistake to make when quickly balancing thousands of vehicles.
This pull request (https://github.com/jamespetts/simutrans-pak128.britain/pull/122) corrects the spreadsheet and .dat file by removing all crew costs from the boats. The crew costs will be assigned to the boat horse, as with other canal barges.
This makes sense - thank you for that. Now incorporated.