A complete Millennium preamp was about $55, direct from Peavey, the last time I got one. If it doesn't already have a Mill pre, you'll also want to order a set of knobs, since you'll need a stacked knob for the Mid pot (and they don't sell just a stacker that matches the knobs that come on a non-Mill pre Cirrus). That'll add another $20 or so.
The Mill pre is a totally worthwhile upgrade (in my opinion), even on a Cirrus that has a working stock preamp. I would absolutely for sure put one in a Cirrus where the original preamp died.
Part #s, prices, contact info is all at the end of this thread:
http://www.talkbass.com/forum/f38/peavey-millennium-preamp-upgrade-step-step-395853/
Just curious. I have a Cirrus Bolt and it has the stacked mid knobs. Would a Millennium upgrade add anything else on mine like a different voicing to to the bass, or is a Millenium pre and mine essentially the same?