I got a pint of Green Light brand 47% MSMA crabgrass killer at WalMart last week. It is potent stuff and a little bit goes a long way. For about $9 you can kill a 12,000 sq ft area of crabgrass. It also kills a long list of other grassy weeds.
But BEWARE! "Injury may occur to Bluegrass and Fescue." Bermuda and Zoysia are ok if well establishied. It will kill Centipede and St.Augustine.
I use it where the crabgrass is dominant and displacing everything else.