It really depends which razors. I found Bic disposeables nearly useless. They don’t pivot, so, unless you have the most acute sense of razor angle as to pertaining to skin surface, they’re either going to nick you up, or not get the hair. I’ve found the cheapest decent shave is the Gillette Pivot Plus. Sometimes you can even get them with the two free Sensors thrown in. As for how long Bics last or the Gillette Good News lasts, that’s tricky. Because, I don’t think they even work in the first place. After one shave, they work horrendous.
I use both the Venus Vibrance and the M3 Power. I don’t know why, but I can use the Vibrances at least eight times before changing the cartridges. But the M3 Power, They don’t last past three uses. I don’t like the M3 Power near as much as the Vibrance. I’m probably going to get another Vibrance handle and use it as well, completely tossing the M3.
If you still want to use M3’s buy the cartridges from ebay. They’re alot cheaper. That’s all I do now. I would never pay retail anymore. Besides, M3 Power cartridges are kind of difficult to find these days even if I did want to buy them at a store. Also consider buying the non-Vibrance Venus, the blue ones. On ebay those cartridges go pretty cheap and plentiful, and it’s going to be a much better shave than the typical disposeable.
Also, if you want your cartridges to last longer, use a gel shave product instead of a foamy cream or a liquid soap. Recently, I’ve experimented a lot and have found that I can indeed get a lot more life out of a razor by using a cheap gel like Barbasol with aloe. Soap wears down the lubricating strip and then the razor doesn’t sit as well on the skin surface. Good luck.