At Sapphire Pointe, a subdivision in Castle Rock, huge beige and cream houses, dubbed prairie palaces by local residents, are clustered along curving lanes and cul-de-sacs. Last week the builder, D. R. Horton Continental Series, was offering a four-bedroom, 4,371-square-foot house for $489,697, $40,000 below the listed price. The builder had added upgrades that included fireplaces; granite countertops; and metal banister pickets on the curving staircase. The master bathroom was bigger than some studio apartments in Manhattan.Many sellers of existing homes are offering incentives of their own. When Mary Ann and Brian Kuklinski put their mocha-colored three-bedroom house on the market in May for $275,000, they knew that a house nearby had languished on the market for eight months. After five weeks and 30 showings, they reached a deal by lowering their price to $270,000 and throwing in their washer and dryer, the refrigerator, and a swing set in the backyard.