I 'think' 1 litre is around 0.75 kilo depending on moisture content.
So, 1 tonne is maybe around 600 to 800 litres.
I think £40 a tonne is a good price.
it can be as low as 400 kg/cum if its bark/fibre and dry as tinder or as high as 1300 kg/cum if its got a grit and sand in it and is very wet. You need to know the volume to compare. Same applies with soil. A tonne of topsoil soil can be anywhere between 0.7 cu/m and 1.3 cu/m . This meant last year the average load size of a curtain side artic (which can take 26 pallets spacewise) was only about 22 pallets because of the weight , and in the winter the lorries were loading out sometimes as low as 16 pallets because it was all so wet.
It really is meaningless to talk about weight in such products, you can only really go on the volume.
Just to illustrate, the dry bulk density of peat can be calculated from the particle density and water content of the peat. Raised bog peat and fen peat, on average, have a particle density of ~ 1600 g/litre. In late summer the pores of the peat usually contain 80% to 90% water. Using these figures, the dry bulk density can be calculated from the dry matter. An analysis of 330 peat soils showed about 3 years ago showed that the dry bulk density can be obtained from the equation:
ρt = 10xTM - 7.2
where ρt is the dry bulk density (in g/litre) and TM = dry matter (%).
And that only applies to the peat in cumbria and scotland. Its different elsewhere. :S You then have to factor back in the moisture content of the compost mix (as it gets wetted up whilst being mixed) when finished and the proportional bulk densities of the other ingredients in ratio.
BUt to get back to the original point, £40 a cubic metre (if that what you are getting) isnt bad because there 14 x 70L bags in a cubic metre, and that works out at £2.85 a bag, or £8.55 for three bags and you frequently see 3 for £10 as an offer in garden centres. But then buying compost as loose bulk is always cheaper, thats why growers do it.