Toney, who recently returned to the Bees line-up after completing an eight-month ban for illegal betting, commanded interest from Arsenal and Chelsea in the January transfer window.
Brentford insisted the striker would not move in the winter but with one year remaining on his contract come the end of the season, Frank has conceded an exit in the summer is on the cards.
"It's quite obvious that Ivan Toney will most likely be sold in the summer," Frank told Danish sports magazine Tipsbladet.
"It can be expensive to sell your best player, but on the other hand, I also know that by the summer, he'll only have a year left on his contract with us.
"We also know what he's worth. I don't think there are many strikers in the world who are better than him right now.
"He's a really good striker who is in his prime footballing age. Personally, as a coach, I would prefer to keep Toney, but one day it could be fun to see him at a top team.
"This winter, we actually had no bids for him, but it would surprise me if there aren't a lot of clubs interested in him."
Toney has hit the ground running since returning to the fold, hitting two Premier League goals in three games.
Brentford, though, are hovering just three points above the relegation zone following their 3-1 defeat at home to Manchester City on Monday.
