12 what? Feet or metres. Also how thick does the concrete need to be. Is it for stock, your car, or will lorries drive over it?
What you need to work out is the volume, as ready mix is priced on cubic metres. The last time I bought any it was £108 per cubic metre. This price is a) 2 years old and b) Isle of Lewis prices (i.e no competition).
Get a quote from a company then ring their competitors. If they quote more, say you've been quoted £x can they beat that. Sometimes you'll get a good deal that way.
Other option is to hire a mixer and get your shovel out!