In a bag vacuum cleaner, dust particles quickly clog the pores, thereby blocking the airflow. Because the air is blocked, the cleaner's suction is quickly reduced, and the cleaner is not very effective.
Dyson cleaners have no bag to clog and this is the reason that they maintain constant suction. All other cleaners drop way below the Dyson's suction, so much so that after just one or two rooms they can have lost half their suction, and continue to drop until a new bag is fitted - at least a month away. This is why 76 per cent of people buy a Dyson vacuum cleaner, after recommendation from a Dyson user.
You make an incorrect comparison between the cost of bags and filters. Dyson has replaced the bag with two cyclones. All vacuum cleaners use filters; for the DC02 we recommend filters are changed every three months, an average annual cost of pounds 20, not pounds 37. We have now developed vacuum cleaners that need one filter changed every year at a cost of pounds 10.