250k per week works out for the full season given say he arrives 11th August as 11.5m total paid.
Signing say a 28-29yo on a three year deal for 20m on 75k per week would cost us per season 10.8m per season with little to no real resale after and that's ignoring any signing on and agent fees which likely would see the cost pushed up to around the 11.5m mark.
Difference being if Grealish is a success we can do a similar deal next season with city, if he isn't we walk away now lasting residual costs carrying over, on the signed player you are paying that 11m for next and the one after also.