A Dyson isn't actually more expensive than most decent vacuums on the market. When you look at the five year cost of owning a vacuum, a Dyson (with it's five year parts and labor warranty and lifetime rinseable filters) will cost you exactly $0 to own, while most other vacuums (bagged or bagless) will add hundreds to the cost of ownership.
One example is the Kenmore bagless upright (Sears # 31040) which sells for $299.99. It needs a new exhaust filter annually ($20) and doesn't come with a warranty ($70 for a 5y PA). Add those up and you get a $470 vacuum which is a bit more than what a Dyson DC17 costs. Of course you can skip the 5y PA and potentially add hundreds more to your TCO if/when it breaks down.