Everyone talks about saving money by switching to wet shaving with a DE. So far I've noticed that before I could use a $2 blade cartridge for 2 weeks. Now I'm using a $1 a week (Swedes or Feathers) blade. I've switched to a more expensive aftershave. My brush and razor together cost about $70. About the only place I've noticed savings is in soap.
Saving money?! Read on, you'll quickly figure out that for most gents here it's not about saving money anymore. Sure, that's often the reason to switch to DE and using a shave soap or cream, it was for me. I went for a long time with Williams, an Ever Ready brush and blades, all of which I picked up in the grocery store.
One thing to consider in your comparison of cartridges and DE blades. I'll bet that you didn't do multiple passes with the cartridge, so that is one reason that they lasted longer for you.