The cost of potting compost can be quite prohibitive if you have a large garden like ours. And peat based compost has environmental concerns too. I make my own compost of course, but it's never enough for my needs, and since I have fairly terrible soil, I do tend to need quite a lot of compost to improve the soil when planting out, just to give the plants a decent chance.
I shop around for the good deals at garden centres and usually find I can get reasonable compost for around 5p a litre. Lidl is often good value, but currently I'm using Countrywide Stores own brand compost which is available at their stores for 3 x 100 litres for £16.
However, for garden use, as opposed to potting use, I get a cubic metre, which is 1000 litres, of Warrior Compost produced by my local recycling centre here in Wiltshire, Hills, and costs £33 including delivery. It's made from recycled local garden waste and is excellent stuff, weed free and accredited by the Soil Association and at just over 3p a litre great value. When I can make enough room by the gate, I intend to have a bulk delivery which will be even better value then I will be able to use it with abandon as a soil improver and top dressing. And I can feel good about it because it's not only cheap but it's green as well.
If you don't live in Wiltshire, try your local recycling centre to see if they do a similar scheme.