You'll pay $63 in parking for 7 days; subtract that from your total, and that's the price you have to beat. And, if you have't found anything cheaper than $500 for your dates, you aren't looking very hard.
If all you care about is total price, it's easy to go lower than that. The hotels you find will not be anywhere nearly as nice as a Disney value resort, but they will be cheaper.
Example: hotelmartusa has the Hawaiian Maingate for your dates for a total of $246.
Expedia has 52 hotels for your dates less than $70 per night, many include breakfast, and all of them will save you at least a few dollars. Some of them might even be nice. On the other hand, you'll need a rental car at all of them, so if you would normally take a towncar and ride busses all week you're back to breaking even (or worse). Even with skyauction, best case, you're going to spend about $225. If you add a rental car to that (and you'll need one) and you would otherwise take ME plus Disney transportation, again, you're back to saving a few tens of dollars, tops.
The reason to stay offsite isn't to spend less in total. If you're happy with a Disney Value resort, and you don't mind busses, then they are very very hard to beat, particularly while ME is free. The reason to stay offsite is to dramatically upgrade your lodging without dramatically increasing the costs.
We travel with small kids, and absolutely require separate bedrooms. For almost exactly the price of a single Value-category room (after AAA discount) during our dates, we are getting a three bedroom/three bathroom townhouse, a 42-inch tv in the living room plus TVs in each bedroom, with premium movie channels and surround sound, a PS2 with a games library, a library of DVDs, a fully-equipped kitchen, private washer/dryer, high-speed wireless internet access, and a small private pool. The property also has a common free-form pool with waterslide and a hot tub, plus a fitness center, tennis courts, and a playground. It is offsite by all of 1.5 miles, and is 5 minutes to AK's parking lot, 12 minutes to MK's. We drive even when we stay on-property, so there is no additional cost.
If I wanted to even approximate that on WDW property, I'd need at least two Value rooms, which would cost me twice as much (and wouldn't have the kitchen, the waterslide, the hot tub, or the private pool. Oh, and high-speed networking is an extra $10/day.)
If I wanted 3BR lodging onsite, I'd need to buy about 500 DVC points at OKW, more at boardwalk or saratoga springs. The 2006 maintenance fees on 500 OKW points would be over $2000, or more than 2.5 times what I'm paying for the townhouse I'm renting, and that ignores the $37,000 and change I'd need to buy them in the first place.
