chesskidnate nailed the problem. Having Cargo-Cargo-Cargo-Cargo-Cargo is not substantially different from having Cargo-Silver-Silver-Silver-Silver. Both hands draw a card, restore your action, and produce $8. So why bother with even the first Cargo? Why not make that a Silver too?
Get +$3 for each discarded Cargo, and then there will be a reason to pick them up.
Sure, you can ask why bother with the first cargo if you're looking just at Cargo and Silver, and it's a fair thing to look at initially, but I'd say that Cargo will do best when you can combo it with cards that let you draw a lot - so Lab, Menagerie, Library if you have Villages -
I think you're missing what I'm saying. I'm saying it's ALWAYS worse than Silver, no matter what the circumstance (barring combos with TR/KC to turn it into a Village, Golem finding it for you, Scrying Pool drawing a bunch, and weird edge case stuff like that).
So let's say you have lots of Labs, and you finally wind up with a hand of 15 Cargoes. Great -- you can play one, discard the others, and poof, you get $28! That's great, but if you'd had 15 Silvers instead, you'd have $30 to spend. No matter what you do, no matter how many Cargoes you have, Cargos can never produce more than an equivalent number of Silvers would have.
Again, with combos like Scrying Pool that take advantage of the fact that it's an Action card instead of a Treasure, you can make them better than Silver. But they need those combos to improve upon Silver, and usually there won't be any available in the kingdom.
Under-rewarding collisions is pretty common, I think, in fan cards. Tournament awards a Prize, Treasure Map puts four Golds on your deck, Baron gives you +$4. So why does Cargo only turn other Cargos into Silvers? Since failure to collide is a cantrip effect (instead of a dead card in hand), you can't go overboard like having four Golds in your deck, but you certainly need an effect that is stronger than what a $3 card would normally just give you outright.
Archetype is quite correct that adding a flat +$1 bonus to Cargo is the wrong way to go, as that powers up the default effect too much -- and then you wouldn't want to discard other Cargoes to a Cargo anyway, because you'd get better use out of playing each Cargo separately (Peddler effect, worth ~$4) than to discard them to another Cargo (Silver effect, worth $3).
If you want this card to work, it has to be better when you collide them and not good when they don't. Mess with either side of that scale, and the card will be over- or under-powered, and/or entail an option that players will never want to take advantage of.