I did an analysis of what Transports in the Shipping Box actually bring way back in AACW. I don't think the rules for this have changed in CW2.
I only took Transport Squadrons into account, because you can only buy squadrons (2 ships), but I'll refer to them as Transports, because I'm too lazy to write squadron all the time
Transports bring not only money, but also WSU (War Supply), depending on how much WSU of which you have on hand and are producing per turn. If your WSU on hand and production is way down, Transport will exchange some of your money income for WSU income from shipping. I don't recall the details, but I don't think you get more than about 1 to 5 - WSU to $, and you won't necessarily get the WSU every turn IIRC, but that might be wrong. I just don't recall exactly.
So how much of which you get is very conditional, and in my analysis I simply counted money and WSU together for simplicity.
There's a very steep Diminishing Returns curve for Transports, which flattens out very quickly. So the first Transport in the box brings just over $20(!!), while adding a second has both bringing almost $16 each.
At 6 it's nearly $10 each.
At 10 it's just over $7.6 each.
At 15 it's just over $6 each.
At 23 it's almost exactly $5 each.
At 25 it's $4.88 each, which is where I quit testing.
IIRC Union NM was about 100, and NM affects the returns was well. The higher your NM, the better the returns.
So with 25 Transports in the Shipping Box with about 100 NM you should get about $122 each turn.
Raiders in the Shipping Box will reduce income greatly. I've experienced in a PBEM game losing upto a quarter to a third of shipping income per turn with between 3-6 Raiders in the box, IIRC. So if there are Raiders in the box, you dang well better send in some hunter-killer squadrons to chase them down