The Premier League transfer window officially opened on Thursday. 

Some of England's biggest clubs might have to wait to do their business as Liverpool, Tottenham, and Arsenal all have the small matter of European finals to worry about first. 

Chelsea, of course, are still waiting to hear whether their two-window ban will be overturned. 

All of the top six - yes, even Spurs - must be giving serious thought to how deep their pockets really are. 

The likes of Matthijs de Ligt, Gareth Bale, and Philippe Coutinho are all being linked with moves to the Premier League, but players of that calibre are not going to come cheap. 

That said, England's increasingly established 'big six' have not shied away from splashing the cash in recent years. 

In the season just gone, there was an alarming gulf between Manchester City, Liverpool, Chelsea, Tottenham, Arsenal, Manchester United and...well, just about everyone else. 

It's little wonder, though, when you take a look at the spending of European teams over the past decade. 

City top the charts 

The chart below, provided by Transfermarkt, shows that City have forked out more than anyone else with an eye-watering £1.36billion. 

All together, the top six have spent £5.64bn in that time. That really is an astonishing amount of money. 

It doesn't make kind reading for the likes of West Ham, who feature in the top 25 clubs across the continent but haven't had all that much to show for it. 

The same could be said about Southampton - really, where has all that money gone? 

For fans of many teams, figures like this are rather exasperating and only serve as a reminder why it's becoming so hard for other clubs to challenge. 

However, the madness shows no signs of stopping. 

Barcelona, who are the highest-ranking non-English club, are expected to bring in Antoine Griezmann for a fee widely reported to be in the region of €120m. 

Elite clubs' spending is not letting up soon.