Wow. You must have strong whiskers. I can get a couple of months out of one insert, shaving every other day. So that's about 30 shaves. I admit the last shave isn't as comfortable as the first but I know when to change. I'm pretty sure I haven't paid anything like $4 per insert, I just wouldn't. I think they are like $6.99 for a 4 pack at Walmart.I'd get ~5 shaves before it dulled
The manufacturers used to send out samples in the mail. Schick, Bic, Gillette. You usually got the handle and one or two blades and a coupon to buy more. I could never throw the handles away after the blade inserts were used up. So about a year ago, I decided to go through the various handles I had. For those that weren't obsolete, I went out and bought insert packs. Stupid thing to do just to get some use out of the handles. But it gave me a chance to try them all out again and decide which I liked best.
In the course of this review, I learned that some shaving products that were introduced in the US wound up in overseas countries after their sale in the US had stopped. So if you have a favorite razor product you can't find inserts/blades for anymore, you might find them on Ebay from Singapore, Thailand or similar.