Toronto goaltending situation is a mess. Campbell is injured and they have to rely on unproven AHL goalies right now. They need a reliable #2 in case Campbell gets hurt again before the playoffs. Allen is signed for another year as well for cheap.
Montreal is clearly rebuilding. They are winning a bit too much right now for their own good. They don't expect to be competitive next year, and Price just started practicing with the team.
Montembeault/Hammond is good enough for the rest of the season, but they can send one down without a problem and absorb a bit of salary. Price/Mrazek is alright for next year, since they don't have to pay for any big new contract before 2023 beside maybe Romanov.
The trade:
Toronto gets: Allen (50% retained)
Montreal gets: Mrazek, Hirvonen, 2022 2nd, 2023 3rd
Toronto gets a reliable 2nd goalie for two runs and save 2.4 millions on the cap for this year and the next.
Montreal gets another prospect and picks for the rebuild.
Fair?