I came across some figures comparing the cost of coal distribution from depot to customer by 30 cwt truck and a horse and cart in 1931. The figures seem to relate to an adequately funded and well run coal business. Two points about the graphs, first they are for 1931 and are not comparable to costs in 2017 and that the original data is in pounds, shillings and pence which was converted decimal pounds for the benefit of Excel. I have doctored the data a little for the sake of comparability. In 1931, the price of domestic coal was between £1.50 and £4.50 per ton depending on the grade, local terrain and market conditions. Anthracite was the premium product whist Bituminous coal was cheaper, also coke from gas works was widely used.
Both the horse and truck were depreciated over four years and funded by money at 5%, the horse cost £90 and the truck £250. The cost structures for both modes of transport is broadly similar, the exceptions are higher capital related costs of the truck, the legal requirements for a license and insurance and maintenance. Food for the horse and fuel for the truck are similar as are the wages of the driver.
The big difference is the level of productivity, the horse shifts 38.5 tons/week, whilst the truck can do 49.5, but the unit costs are similar at around £0.20/ton. I suspect that there was a lot of variation within the industry. If only one man was employed to work with the truck, he would have to work harder than the bloke with the horse and cart, the references I have seen to coal sacks at this time suggest there were 1.25 cwt ( very roughly 62kg or very heavy, I struggle with 25kg bags of sand). This might have been OK for a youngish man shooting coal into a cellar with street access, much less for an older one shifting the bag from the street to coal store in the scullery at the back of the house.
Some random reading suggests that the domestic coal market was split into three sectors. At the top end would be customers that bought coal in large quantities, say greater than half a ton, possibly belonging to a "coal club" which spread the cost more or less evenly over the year, trucks would give an advantage to merchants serving this group. Those serving customers purchasing less than half a ton and paying the current market price might have a cost advantage from the potentially lower costs of the horse and cart. At the bottom end of the market would be those purchasing small quantities of coal, possibly as little as 7 pounds would pay high prices to men with barrows.