The night of the week doesn't factor in the band value calculation.
It goes something like this:
(Sales - costs, with no band) vs (Sales - costs with band on). Monday sales have no influence on the sums for Friday. ''Keeping the lights on'' costs are fixed. Product costs are relative to sales. Band cost is ( usually) fixed.
Staff costs really should go up with band, or they are paying too many staff the rest of the time and it's not a factor. Bar is quids in if they get the regular staff running when a band is on.
For the sake of making the numbers easy let's assume staffing is always commensurate with the level of trade. The cost per pint is less when they are busy because the ''keep lights on'' costs are the same every night.
'TimR's Monday night' doesn't hold to the staff commensurate condition. The busy nights effectively have a portion of Monday night's staff and 'lights on' overhead attached to them. But those costs are fixed whether the band is there or not.
Some places make a go of having a top notch band on Fri Sat and a fool on a stool or a quiz or poker or something the rest of the week. Those are the gigs every band wants but they only go to the ones that keep the punters happy. The bar expects to be busy. The band expects to be paid.
More complicated is the bar that has occasional bands vs football and juke box barman. Costs per pint on a busy band night is then very much in the bar's favour, but it has to be, to ''keep the lights on''.
So in a weird way Tim is wrong but also right.