It depends on what ph your soil is how much you should apply. We do ours to try and control the buttercups, but we have a very small field (under 1/2 acre). 3-4 bags is enough for ours, and we spread it by hand as it is such a small space.
If you search on here for liming, there are quite a lot of useful posts. I saw one quote saying they had been told £260 for 5 acre.
Beth