I’ve never really bothered with special cargo since sourcing and selling vehicles was always the best ROI option.
However I only found out today that the only way to fill your office floor with banknotes is by selling $20 milion worth of crates, so I bought two warehouses today and am slowly but surely filling them up. Will take a while to reach that 20m
Apparently the biggest warehouse (111 crates) sells for a little over 2m. I've got two large warehouses, so that's 5 times each.
Since you can only source 3 max at the same time I'm looking at a maximum of 370 sourcing missions. (though sometimes the three crates are together in a vehicle, so actual amount of sourcing missions will be less)