Grealish has been heavily linked with a move away from Villa Park this season after a fine return to the top flight with his boyhood club.
"It's the going rate for players in and around but for £80m I want someone who's going to start week-in and week-out," Warnock told Stadium Astro.
"When you think about his global appeal for Manchester United, is he going to sell you shirts week-in and week-out to repay a lot of that money that he's going to cost? I'm not so sure he will.
"But who else will buy him for £80m with the pandemic that we're coming out of and can clubs afford to shell out £80m?
"I don't think they can. That figure would have to come down considerably and I think it will as well."