This goes through the JSON API for reputation amounts for questions and halved the total gain points. The loop is needed because there is 90 days limitation on the range.
import sys,time,urllib2 as u2,json A=sys.argv[1:] U,S=int(A),A[1:] and A or "stackoverflow.com" t,c,g=0x1ca55fc00,time.time()*1000,0 while True: u="http://%s/users/rep-graph/%d/%d/%d"%(S,U,c-t,c) d=json.loads(u2.urlopen(u).read()) if not d:break g+=sum(x['Gain'] for x in d if x['IsQ']) c-=t time.sleep(1) print "Your reputation is expected to be down around",g/2,"points",
Example for Jeff
You can specify the site name like superuser.com, serverfault.com like
filename.py 1 superuser.com
As a result, Jon Skeet is going to lose 688 points, and Jeff is going to lose 2286 points
PS: I noticed that above numbers are not divisible by 5, reps cap is the only I can think of for that case.
Let me know your ideas about bugs or improvements.