We have Polynesian for late June with the current AP discount and it comes to $259 including tax. I think it would be slightly cheaper in August but should still be around $230(AP discounts are out until 9/30).
I think the absolute best bang for the buck is Swan/Dolphin. They have many fabulous discounts and you should be able to find something in or below your price range. At Swan/Dolphin, you have easy access to Epcot/MGM and BW, plus all the amenities of a deluxe, which it truly is.
Is anyone in your party a teacher, nurse or any type of government employee? If so, you can get book up to 3 rooms at a price of $129 each. If you don't qualify for that discount, there are also 50% off AP, entertainment and I believe DC discounts.
If you're set on Polynesian, but can't afford it, WL is the next best thing. Close proximity to MK and just as much themeing as Polynesian for a more reasonable price.