Ndiaye left for Manchester City on deadline day in a deal worth up to £65 million, comprising £60 million guaranteed plus £5 million in add-ons, ending a two-year spell in which the Senegal international scored 21 goals in 74 appearances and became one of David Moyes's most important players.
Coleman, who retired at Everton this summer after 17 years at the club, could not hide his frustration at the fee.
"Time will tell. Jack Grealish did well when he came in last season, but the reason why I did that," Coleman said on the Stick to Football podcast, shaking his head, "he gets you up the pitch, there could be three people around him and he gets out of it. Once he adds goals and assists... but the way he works, he had a spell in the lower leagues, he's hungry, can take criticism and keeps going.
"I'm just surprised for someone so good — I'd have liked it to be for a bit more money when you see the price. Nowadays, it might be a low price. He was really good for us."
The comparison looked notable elsewhere in the market, with Aston Villa paying £65 million for Chelsea's Nicolas Jackson despite a difficult spell at Stamford Bridge and a loan at Bayern Munich, while Nottingham Forest paid an initial £45 million for fringe striker Liam Delap after just three goals in 45 appearances for Chelsea.
Coleman was warmer discussing Jordan Pickford, crediting the goalkeeper as indispensable to Everton's Premier League survival over the years.
"I do think Everton would be in the Championship without him. When he came in, he was young, immature, but he's so professional. He plays every week, he's never injured, switched on and focused," Coleman said.
Asked why Pickford has rarely been linked with a move away despite his performances for England, Coleman pointed to genuine contentment rather than a lack of interest.
"He does love it at the football club, he loves being loved and the fans absolutely love him. I don't think it's something that's ever come about, but I do know he absolutely loves it at Everton, they love him, they idolise him."