Hi Ladbrokes
It depends partly on how big the sacks are, if 70 litres that is a fair price, although more costly than buying a three ton load or whatever the other deals are. I would go for a larger amount (if you have a trailer to collect it, the price is low) and spread some about an inch thick on both beds and paths.
Pathways are important too as vegetables, plants generally send roots further than one imagines and helping them to feed in paths is a good way to keep the soil structure open and good for walking on. My paths of clay are 13 years old now and still have aerated soil, after all the passage of feet and wheelbarrows.
Good luck with growing this year.