Essentially this is a case of downgrading some of your consumable parts, without sacrificing performance.
Consumable parts are components such as chains, cassettes, chain rings, brake pads, tyres and tubes - items which wear out over time and need replacing.
Personally I run 8 speed on some of my bikes, as it is very reliable, easier to setup, requires less maintenance and very cheap to replace. A new 8 speed chain costs approximately £8 and cassette would be £15, which is far less than the higher geared alternatives.
Buying these items in bulk when they are on sale is the easiest way to make cycling cheaper. Stock up on tyres and tubes when they're half price; you might not need them now, but you will in a few months.
This method also has the advantage of never being stuck without a usable bike, as in time you'll have a box of spares ready to go.