I've been pressing 5g at a time, bottle tech style, and I actually find I have
higher yields with the bags then without. I'd say an increase of 1-2% on avg. I attribute this to the "puck" being kept to a smaller size, the reduced expansion means the rosin has less material to travel through, and less gets trapped; in other words less is trapped in the bags then is trapped in the increased surface area caused by the flower pressed without a bag.
And at 1-2%, and guessing $1 per bag, based on 5g presses, I'm actually ending up in the plus.
eg. My typical yields are in the ~25% range ... so 1-2% increase (ie. from 23-25 or 24-25 ... my yields range typically these days from 24-29%) means a 1/25 to 2/25 % increase in yield vs yield without bags. ie. That's a 4%-8% increase in end amount of rosin. Even at $150/oz and assuming just the 4% extra, that's still $1.07 worth of extra rosin
(and I'm paying more than $150/oz and I'm getting more than the 4% typically ... in other words the bags pay for themselves x2 ... maybe even x4)