most light beers have somewhere in the neighborhood of 100-125 calories (Millerlight, bud light, coors light, etc.). however the difference in how much crappier tasting mgd 64/bud select 55 is than normal light beers is not worth the extra 50 or so calories IMO. plus, if you're drinking beer to get drunk, you're going to need to drink so much more of the super low cal beer to get drunk anyway that your caloric intake basically comes out a wash with if you'd just been drinking regular light beer in the first place.
that being said, if you're drinking beer just because you like beer and not to get drunk, then why in the hell are you drinking light beer to begin with?
if you want to drink on the low cal, start drinking wine or liquor. much easier to play the calorie game with these than it is beer. if your dead set on beer, cut out something else in your life (fast food, soda, etc.). drinking crappy beer on a regular basis is miserable, just ask any homeless man or hawkeye fan