If you're talking that sort of money, you can either get someone famous, or someone talented.
The question is, do you want to give your audience a fantastic musical experience, or do you just want to draw people in with a big name? They'll be more likely to come back for future events if you do the former.
I speak from experience here; one of the venues I work at has "famous" headliners every week (DJs, not pop artists), people like Oliver Lang, Brandon Block, Foamo, Jonathan Ulysses etc. And unfortunately, they just play track A into track B and it just sounds like EVERY OTHER DJ out there. To me, it's boring and does the audience a disservice; if I was booking DJs for an event I'd want people like Moldover or Bass Kleph, people who can do live work and give the audience a completely unique performance; something they can't get just by listening to the latest Ministry of Sound album.
Personally, me, Patch and a couple of others would be happy to split £6k between us for an hours' set each ;)