Well, the basic math I do is that it takes me time and fuel to go get it, time to unwrap it (because its usually individually wrapped...) and then time to feed it and dispose of the wrappings.
So a loaf of bread is usually somewhere between a pound and a pound and a half. Right now feed costs for a balanced ration are about $0.19/lb -- $380 a ton. and at $0.19 a pound the feed mill delivers it to my door and puts it into the bin.
So to make economic sense a loaf of bread has to beat $0.19 at my farm gate. So lets do the math:
20 miles to pick it up, 20 miles to bring it back, 2 gallons of diesel, $8
1 hour of driving time, $12
Insurance, wear and tear, $2 (.10/mile)
Total $22 (= 115 loaves of bread)
Time to load and unload depends on the number of loaves. Lets say I get 200 loaves. Figure I can toss them into the truck pretty quickly; 15 minutes. Unloading about the same, 15 minutes. $6 (=31 loaves)
Unwrapping takes some time. lets say I can unwrap a loaf in 5 seconds average. Unwrapping 200 loaves would be (200 * 5 / 60) 17 minutes. Disposing of the waste will take some time, too. 15 minutes. So that's another $6 labor (=31 loaves)
So for this particular trip, I picked up 200 loaves. out of the 200 loaves I used 162 of them to cover the cost of the trip, leaving 38 loaves "profit". How much did I make? about $7.
that is, I saved $7 off of the equivalent weight of feed. Except that loaves aren't a complete diet, a pig doesn't live on bread alone, so I'll use them to supplement the pigs diet. It stretches my purchased feed a while.
My rule of thumb? Big quantities only. I don't pickup bread unless there's at least 200 loaves, but if I can make one trip and pick up thousands, even better!