The Villa talisman has been on fire this season, with seven goals and 10 assists across competitions, and Merson believes he would be a great fit at Camp Nou.
"I think that much of him," the Arsenal legend told Sky Sports. "He's one of the best around and if I'm a manager at one of the big boys - and no disrespect to Aston Villa here... I'm looking at £120 million, £130 million for him, I would pay."
Merson added that after Barcelona's 4-1 thrashing by Paris Saint-Germain during the midweek, Grealish would be the ideal player who could add some much-needed firepower to Ronald Koeman's side.
"I watched Barcelona the other night get ripped to shreds," he said. "If I'm Barcelona, I'm buying him tomorrow morning. It doesn't matter how much it is, I'm buying Jack Grealish - because Jack Grealish at the Nou Camp would absolutely rip it up."
However, splashing that amount of cash on one player might not be feasible for the LaLiga giants, who are struggling with debt and are looking to offload a number of high-profile players.
Grealish, meanwhile, has his sights set on a place in Gareth Southgate's Euro 2020 squad. He told BT Sport: "I just think about it all the time.
"I'm just desperate to go to the Euros. I just pray that I can carry on performing, carry on impressing the manager and fingers crossed. When I got into the first (England) squad that was my only thing that I wanted to do (go to the Euros) and I'll literally do anything to go to the Euros."