thezeb.com has w3's for 139 and they retail around 200. 179 for WO's is as crap tacular as you can get. You will have to email Jay at the zeb for all the prices (Because they cant give prices on their site for JL equipment)
W0s may not meet a competitors standards but thats why they don't claim to be a competition sub or cost as much as one either. They are excellent value (sound for money). They play bass notes with accuracy and are plenty loud.
Sure, the W3s are better than the W0s, that's why they cost $50-$60 more per sub.
The W6s are a more capable sub but with the wattage available are not needed. W0s, regaurdless of their low power rating, can handle 200W apiece and are more efficient than W6s.
If you're strict on a $500 budget, it may be wiser to go with good quality components one at a time rather than settling for an entire system. I would go with a 4 channel amp and good interior speakers first before the sub and sub amp. If you're just into how hard your car can pound, then go with MTX Thunder 7000 (last year model) which can be bought for dirt cheap, and either a rockford amp or MTX amp. They discontinued last years models which can be found at clearance prices.