I just upgraded mine in the e36
This is all cdn prices tho...
I paid $650 for Alpine Headunit
$450 for Infinity Kappa 505cs 5/14" components
$180 for Infinity Kappy 4" two way speakers
$500 for Audiobahn 4x75 watt amp
$700 for audiobahn 2x400watt amp
$300 for 2 12" Cerwin Vega vega series subs
so for just upgraded speakers you could easily spend less then $1000 US
MBQuarts are good speakers, as are focal.
Check around online.
http://www.sounddomain.com has a good selection of speakers to give you an idea of prices.