Cellino told Gazzetta dello Sport (h/t Football Espana) that the Italy international is a wanted man, although he also made it clear he is keen to keep hold of the teenager:
"All the big Italian clubs are interested. Abroad there’s Atletico Madrid, Paris Saint-Germain and Manchester City, but in my view, he'd be better off remaining as a protagonist with Brescia.
"The other day, his agent and his parents were telling me about this €50m evaluation. I replied that for me he is worth €300m, which means I don't want to sell. I have a dream. If Sandro accepts, I am ready to make a big financial sacrifice to extend his contract."
Tonali has come through the ranks at Brescia and helped the club win the Serie B title in 2018-19 and achieve promotion to Italy's top flight.
The teenager has started all eight of his side's league outings this season and scored his first Serie A goal in Saturday's 3-1 defeat at Sampdoria with a brilliant free-kick, as shown by Premier Sports (UK only):
Tonali's performances for Brescia have also seen him called up to the senior Italy side. His first call-up came while he was still in Serie B:
The teenager won his first cap for the Azzurri in October. He made his debut as a substitute in their 5-0 win over Liechtenstein in Euro 2020 qualifying in October:
The central midfielder has drawn comparisons with Gennaro Gattuso, Andrea Pirlo and Daniele De Rossi and offered his thoughts on his similarities to those players after that match.
He told Rai Sport (h/t Football Italia): "It's tough to pick one of those champions. Maybe a mix of all of them would be perfect! I guess there are similarities with Pirlo in a way, but I also put a lot of grit in there, so maybe more Gattuso."
Serie A side Fiorentina made an offer for Tonali in the summer transfer window but will not renew their interest in January. Director Daniele Prade told TGR Toscana (h/t Football Italia) that Brescia would not want to sell their starlet midway through the season.
He said: "No, we won't try again in January. We'll be concentrating on sales rather than purchases. Besides that, why would Brescia want to sell Tonali mid-season?"
Yet Brescia look destined for a battle to keep hold of their talented teenager. He only turned 19 in May but has adapted well to life in Serie A and looks to have a big future ahead of him.
Cellino may be adamant he is not interested in selling but Manchester City and PSG are clubs with the financial power to make an offer he may find difficult to turn down.