A report from the Royal Institute of Chartered Surveyors earlier this year worked out that the average price of land in Scotland in 2014 was about £4,375 ish per acre. (
http://www.bbc.co.uk/news/uk-scotland-scotland-business-31146605)
An article in the paper yesterday said that prime agricultural land would be about £10,000/acre and secondary land between £5,000-7,000 per acre.
I think Lord Flynn's estimate would be about right.
The other idea of finding an idea of land prices is to look at any farms that are for sale in the local area and work out the price per acre from the schedule especially if there is 'lots' of only land.
At the end of the day it also depends on the land owner and how greedy they are.
I know of one farm in Angus who sold 2-4 acres of very good agricultural land to a local company and they sold it to the company for £5,000 per acre. A bargin!