Neymar and Lewis Hamilton went out in London together on Monday -- in what turned out to be a VERY expensive battle for fashion supremacy.

See Neymar's red jacket? Yeah ... that's a Louis Vuitton x Supreme bomber varsity piece that's being sold on one fashion site for $23,000!

Hamilton was sporting a bunch of designer stuff that hasn't even been released to the public yet --including a Balenciaga sweatshirt and special gold Off White high top sneakers. 

The two hit up Zuma restaurant together before rollin' over to the famous Cirque Le Soir nightclub. 

Wonder who paid ... considering Neymar just signed a massive deal with Paris Saint-Germain that's expected to pay him more than $265 MILLION! 

Hamilton reportedly made $46 MILLION just last year. 

Yeah, it's good to be rich ...