How Much is "Safe" To Carry Out of Jita In a Freighter?

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:

The 1 billion collateral is because that is just below the profitability threshold of hisec suicide gank fleets.

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:

• Tornado with 1400mm T2 Artillery:
  • 11-12k damage every 13.7 seconds (overheated)
  • costs around 125 million (estimate from Pyfa)
  • ~ 1 shot in 0.9-1.0 space before CONCORD, maybe 3 shots in 0.5 space
• Freighter EHP: 160k

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 is possible to gank with cheaper ships (though it's more effort to get enough Catalysts 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."