Amazon has 10.06-Lb Scotts Halts Crabgrass & Grassy Weed Preventer (5,000-sq ft) for $19.47. Shipping is free w/ Prime or on $35+ orders.
Note, there may be delivery restrictions based on location.
Features:Prevents the germination of crabgrass, foxtail, spurge, barnyardgrass, and more as listed in the early spring all season long, and moss (except in California), Poa annua, henbit, corn speedwell, and chickweed in the fallApply this pre-emergent weed killer in the early spring to a dry lawn or in the fall; rain, snow, or freezing weather after application will not affect the performance of this productThis lawn care product is for use on all grass types (with the exception of Dichondra or Bentgrass)