When I leave my blades in a light coat of baby oil, I get roughly 10 shaves per blade....So a 5 pack of DE blades based on me shaving every second or third day, lasts me anywhere from four to six months depending on the brand.
It depends on the brand of blade, how many passes you make on your face, whether you shave your whole face or around a moustache etc, whether you also shave your head with it.....
In other words, there are a lot of variables.
That said, shaving only my face, I get 4-5 shaves which are at least comparable (although more pleasurable for me) with a cartridge shave. So I could for example buy 100 astra blades for around 10 US dollars, use two a week, and have them last me for almost a year.
Some blades are cheaper, some more expensive, but even at one shave and then in the (safely wrapped) trash, 400 blades would be only around 40 dollars.
Plus 1 Bluebriz...
I only use a DE blade a max of 3 x's because they are so inexpensive.
Whats the Dorco like?
I have never used a Dorco...if you find unbranded blades at Walmart or Walgreens, usually they are Dorco's.
They don't get a lot of love...or so my friends say.
My top 5 blades are:
Polsilver
Gillette Silver Blue
Voshkod
Astra SP
Israeli Red Personna
I trash my Feather blades after 3 head & face shaves (3 passes each). I have a goatee. I could probably get 1 or 2 more shaves out them, but I don't like to chance it, and it's an easy decision given the price.
I tend to stick to 2 shaves per blade.