The kind you buy every year or every two years will end up costing way more than a vacuum that will last 10 years to a lifetime.
The best example of this is in an all plastic vacuum you spend $300 dollars on that will last approximately 3 years. That is an average cost of $100 dollars a year. As opposed to a vacuum that costs $500 with mostly metal construction that lasts 10-15 years or more at an average cost of $40-$50 per year. Alot of people who visit our store have bought as many as 7-10 vacuums in a ten year period, and are fed up with plastic junk.
You will be alot better off to buy one quality unit that performs many functions(vacuuming carpet, hard floors, shampooing)and lasts 15 years, than two or three low priced units that will need to be replaced every two years.
I personally have had the same unit since 1996 and I have many customers who have had top quality vacuums as many as 50 years.
moral of the story
"buy the very best once and you will not be sorry"