That outlay will dwarf City's current largest purchase - the £61.2m they paid Benfica last September for centre-back Ruben Dias.
The likes of Riyad Mahrez, Aymeric Laporte, Joao Cancelo, Rodri, Benjamin Mendy and John Stones aren't far behind though, each having cost Pep Guardiola's side in excess of £50m apiece.
Ahead of Grealish's arrival in Manchester, Daily Mail - per Transfermarkt - has broken down exactly how much the Spaniard has spent on transfer fees since becoming City boss in 2016. The total amount, which includes the nine-figure sum that the club will splash out on Grealish, is staggering.
Granted, Guardiola has enjoyed great success during his time in the hot seat at the Etihad. A haul of three Premier League titles, the FA Cup and four League Cups across the 50-year-old's five seasons in charge is nothing to be sniffed at.
However, when you consider that the club's Abu Dhabi owners have now allowed Guardiola to spend an eye-watering £918m on players, expectations when it comes to trophies are understandably high.
As part of the overall spend of £918,017,363, here are the top 10 most-expensive signings that Guardiola has made as Manchester City manager.
1. Jack Grealish (Aston Villa) - £100m
2. Ruben Dias (Benfica) - £61.20m
3. Riyad Mahrez (Leicester City) - £61.02m
4. Aymeric Laporte (Athletic Bilbao) - £58.5m
5. Joao Cancelo (Juventus) - £58.5m
6. Rodri (Atletico Madrid) - £56.43m
7. Benjamin Mendy (Monaco) - £51.75m
8. John Stones (Everton) - £50.04m
9. Kyle Walker (Tottenham) - £47.43m
10. Leroy Sane (Schalke) - £46.8m
One thing that immediately stands out from this list is that it doesn't feature an out-and-out striker. Despite a spend of close to £1 billion, Guardiola has struggled to find players to effectively lead the line for City.
Gabriel Jesus cost the club £28.8m in 2016, but City's pursuit of Tottenham's Harry Kane this summer proves that Guardiola doesn't see the Brazilian as a suitable first-choice frontman.
If Kane can be prized away from Tottenham, it seems certain that City will need to improve significantly on the fee that they have negotiated for Grealish.
A £100m-plus spend on Kane over the next few weeks would take Guardiola well beyond the billion-pound mark in terms of his spending at the club. In the scenario, the pressure would really be on for the man in charge to finally land the Champions League trophy for City.
Jack Grealish signs for Man City (The Football Terrace)News Now - Sport News