Mark and Joe on Seeing Red speculated about $80-100 per season. But that was just a guess. Any higher and you may consider signing up for a cheap season ticket to get access, then reselling the rest of the tickets.
I guess that's in the ballpark of being reasonable, probably even a bit much, especially if they start that high. Paramount+ is 60/year (ads) or 120 (no ads) and includes Champions/Europa League, Serie A, and a lot of USNT games, in addition to various movies/shows. Peacock is the same with Premier League/NBC content. ESPN+ is now 84/yr (La Liga, Bundesliga, assorted other stuff).
My guess is they'll start at 5/month and inch up over time if successful.