I wouldn't expect one. Those usually happen when you already paid for a thing; for example, if you buy an indie game and they decide to overhaul it into a technically new game, they might offer a discount for people who bought the original product.
Its a privilege to have the opportunity to play in the beta, not a commendable self-sacrificing act.
If someone disagrees please give me the bannerlord beta key. Further more, you probably get a forum badge anyway.
I see the side of both cases. On the one hand you got the beta for free, therefore it was an opportunity for you to play as others couldn't. But on the other foot, the beta is there to help find issues in the game, and you're in theory volunteering your time to play the game and find bugs.
Whatever side you stand on, I'd say that there wouldn't be a sale, especially with the amount of ppl that have been given keys.