If you have enough keycodes on your BCM50 hardware that is on hand then the cost of the upgrade license is about $600. (Can shop around for better prices though)
As the name implies right around 50 sets is about the maximum number of sets for the BCM50. And depending on what you have now if it's not all IP sets that also can mean an expansion cabinet or two to hold a DSM32 card and a trunk card. The base unit can do 12 digital sets, then 32 more on the DSM32 card in an expansion cabinet for 44 in total. If all your trunking is through SIP you wouldn't need another expansion for a PRI card and there are still 4 analog trunk lines on the base BCM50 unit for an emergency 911 hard line.
Part numbers for BCM50 R6 upgrades are:
NTAT0162 BCM50 upgrade from version 3 to version 6. Includes Upgrade DVD, and documentation kit
NTKC1240 BCM50 upgrade from version 3 to version 6. Paper authorization code. Software and manuals downloaded online.
NTKC1246 BCM50 upgrade from version 3 to version 6. Electronic authorization code. Codes sent via email direct from Avaya after order is processed by distributor.
If you need to upgrade from BCM50 Release 1 or 2 then you need to first order the BCM R1 to R3 upgrade keycode. (So it will cost more for an older BCM50 to upgrade all the way to 6.0)
NTKC0935 N0146690 BCM50 1.0 to 3.0 Upgrade Authorization Code
NTKC0937 N0146692 BCM50 Upgrade Electronic Authorization License 1.0 to 3.0
The DVD version usually costs more and you can just download everything from the Avaya support site. (Could even do it before you order anything if you want to read the manuals first)
The paper license is the fastest to get if your dealer has it in stock. Electronic takes Avaya about 2 days to process the PO and email things back to you plus any delays the dealer/distributors order desk might also put on the process.
Then you need an Avaya partner to go into the KRS system to actually generate and retrieve your keycode file for you so make sure you specify that service up front if you do not have a login for KRS yourself and were going to do the software upgrade on your own.
Now if the plan was to do a migration of keys from your BCM400 to BCM50 I'm not entirely sure that Avaya supports that. They generally consider a migration to be from a 400 up to a 450 so I don't think they will do a keycode migration to a BCM50. At least, I have never tried, never asked about it and I have never heard of anyone doing it myself. Most doing the migration want to maintain an investment in media bay modules so wouldn't be interested in a BCM50 anyhow.
And then there is this document from Avaya that lists all the migration options and it doesn't include the option of going from BCM400 down to a BCM50 in the list.