I know that it all varies on who is sitting at the gate camp, but in general, what is considered a safe upper limit of total item value to carry out in a freighter?
The well-known Freight Corporation Red Frog takes contracts up to 1 billion ISK, citing that value as the threshold where suicide ganking a freighter becomes profitable:
Being below that limit won't prevent any goons from blowing you up for fun, but it should stop suicide gankers that are after the money.
A very rough estimation of the cost of a suicide gank would be:
So it would require very roughly 15 Tornados in Jita, costing 1.875 billion ISK.In 0.5 sec space a third of that might be enough, so it would require only 5 Tornados, 625 million ISK. As only half of the cargo drops statistically, you would have to carry 1.25 billion worth of cargo for that to be profitable.
This is a very rough calculation, it might be possible to gank with cheaper ships (though it's probably hard to get enough Thrashers in one place to gank a freighter) and the 3 shots in 0.5 are just an estimate.
Safety is relative. The simple answer is: "However much you can afford to lose is the amount that it's safe to carry out."