Hi Talia Are you sure about those calculations? I have 325 sq meters of allotment (15 rods) approximately and 2 cubic meters of compost gave me a dressing on around 250 sq meters. So if you are using card twice that much would give enough, even 3 times would only be 6 cubic meters at around £75 per bag delivered ( depending where you are but cheaper for a loose lorry load) that is well under £1000.
If you are not sure perhaps order a load and see how far it goes?