This is a question you see a lot on the forums and I wanted to give other CMDR's a perspective from someone that has wrecked a lot of ships and lost a lot of money doing so.
I have come up with a formula to help you determine if the extra profit is worth running without shields. The calculations I came up with are generic but you can input the numbers from the runs you are doing to determine for yourself. Here is the formula:
[cost of rebuy + (total tonnage of ship without shields * cost of cargo per ton)] / (profit per ton * extra tonnage gained by running without shields) = Number of runs needed to cover loss if you loose ship and cargo (round up)
That is all real confusing so here is an example for a T6 (as people like to run T6's without shields). Rebuy costs are based generic builds.
(150,000 + (112*10000) / (1000 * 8) =
1,270,000 / 8000 = 159 runs needed to cover loss of cargo and ship combined.
You can take this further. Average run takes about 6 minutes one way.
6 * 159 = 15.9 hrs of game time. If you loose a ship before then you have wasted that game time.
Try it out. Plug in the numbers from your route. Here are some of the numbers I have come up with. The only ship that it might be worth it is the Sidewinder (15-30 runs depending on cost of cargo and profit):
T6:
130-170 runs
13-17 hrs
Cobra:
120-180 runs
12-18 hrs
Asp:
185-260 runs
18.5-26 hrs
Hauler:
30-60 runs
3-6 hrs
In Solo? Yes; Less weight, longer jumps, quicker turnaround. In PvP I'm not so sure.