Avid upgrades and crossgrades are a bit of a minefield. You're really better just looking up the deals on their site and if you're stuck, taking a look at the DUC forum.
I can say with fair confidence though that any AVID upgprade or crossgrade will be more than $50.
With buying a 'used' license on ebay it can be tricky.
Avids deal is (or at least used to be) that the software wasn't for sale or transfer.
Their official transfer forms were for hardware, which included the software package.
That meant that to sell your copy of PT7 or whatever, you had to sell your mbox.
It also meant that the transfer was full and complete, meaning that the new user had full future upgrade entitlements and what not.
I, for example, have a license code for PT6.9,7,8 and an ilok for 9.
As much as I'd love to, I can't sell you any of my licenses because they're all forever tied to my current PT9 license and to my account.
There used to be a loophole; I'm not sure if it works any more, but.....
You could have owned PT7 for years but just decided to register it today.
As far as AVID are concerned you bought it new today, so they give you all the free amnesty upgrades to bring you up to date.
If you never registered your product and you don't have a conscience, it's worth a look.
Hope that's useful.