Curious as to the community's experience is with last minute upgrades from Y-J on RJ.

I've either not been in a position to upgrade my RJ revenue flights or have flown biz with miles but will have a lot of upcoming travel on RJ over the next few years, but last time at the Amman airport overheard a passenger get offered to upgrade for $700 USD for AMM-JFK. How common is this and are offers ever sent online?

Every time I've flown RJ their business class is at least half empty so I assume upgrades are rare - if anyone here is RJ silver/gold or oneworld ruby/sapphire any data points would be greatly appreciated!