Hi Hempie
Given the height of your proposed raised beds, one solution could be to make Hugelkutur beds, where the base is filled by larger logs, then smaller branches, twigs and leaves, then above that a more traditional lasagna set up using cardboard, grass cuttings, fallen leaves, some manure, some straw (if available), any spent crops (like potato or tomato stems), any green manure available (stinging nettle, comfrey, etc), adding in what compost you have to provide the soil life necessary to promote in situ composting. The lasagna should turn into a usable bed within 6 months, whereas the logs will release heat and nutrients over a several year timescale of breakdown.
It is probably cheaper than 16 cubic metres of pure compost, at least based on UK prices….it just depends what you have available where you are.
I have not done it, but plenty of videos are out there by people who have.
If you wanted to create 16 cubic metres of horse manure-derived compost, based on my experience you need around 70-80 cubic metres of fresh horse manure+straw (which compresses and rots down over 8-10 months (last November to now) and can then break down further in the new beds over a winter). That is quite a delivery!