So by posting this, I hope the publishers don't catch on and fix this.

If I buy the COD Ghosts for the PS3, then paid $10 for the PS4 digital upgrade, I can probably sell the PS3 version ($30 trade-in or probably $50 on ebay). Theoretically, I can get the digital copy for about $20 net.

PS3 version is 50 or 60? So if you paid 50 and then another 10 you would have paid at least 60 dollars. You would then sell your PS3 version for 30-50 dollars. 60-50 (max) = 10 dollars spent for PS4 digital. I don't see any money making in this deal but a potential for a cheap digital copy on PS4.

unless the deal is just digital

Yeah, you're right.  I changed my thinking.  Duh

People could do the same with Vita PS3 cross buy but it was never a problem.

That's a little different.  PS3 to Vita is a slight downgrade (arguable).  However, PS3 to PS4 is a significant upgrade.

PS3 to PS4 costs extra money. PS3 to Vita doesn't.

It balances itself out.

PS3 Vita cross buy isn't even the right comparison to PS3/PS4 upgrade.  Vita is more an extension to play the PS3 version on the go.  I suppose you could use the Vita version as the primary version to replace to the PS3 version but who did that?  And even if this was true, who owns a Vita and buys games for the Vita?  The number of possible people that can exploit this is very small.  Anedoctally I only know 1 person that owned a Vita but sold it after couple of months because he never used it nor had any games to play.

However, lots of people is going to own the PS4.  Also, the PS4 digital copy is a permanent replacement for the PS3 version.  Why would anyone go back play the PS3 version if that person has the PS4 version?  

Yeah, we'll have to see exactly how these "upgrades" really work.

Anyways you will see people who are going to buy the PS360 versions and are not intending to go next gen anytime soon, so the Digital Code will be sold for the same amount 20-25$..