I use a Gillette mach 3 and I found long ago that it is significantly cheaper to buy razors on e-bay.
I can get 10 razor cartridges for $16.99 with free shipping.
Gillette Mach3 Men's Razor Blade Refills, 10 Count | eBay
That's about $2 a month for a high quality shave.
But, make sure they are the real deal.
If they say "blades that fit mach 3" or "blades for mach 3" and don't show a Gillette package, they are not OEM and are generic copies or knock-offs
They are still decent, but are much cheaper i.e. 24 blades for $17.
Generic Replacement Blades Cartridges for Gillette Mach 3 Shaving Razorã€US】 | eBay