There is always an optimal point when it comes to launch vehicle size for $$$/lb to LEO.
- Make the launch vehicle too large, and it will be very expensive and fly rarely
- Make the launch vehicle too small, and you have to launch too often, and with tiny pieces
Personally I think 40-50mT is a pretty good point - especially when it is made up of the same components of smaller launch vehicles. The more frequently you fly a booster, the more confidence you have in it, and the cheaper it will become due to mass production.
BTW, the propellant cost of a rocket launch is just in a tiny fraction of the total launch cost of a rocket. (1-2%, or even less) The hardware and man-hours is what makes up the vast majority of the launch cost.