My cheapest round, as a foreign guest, was Fort Augustus, south of Loch Ness, Scotland, in 2007:
7,50 GBP for nine holes included the pull trolley and the most hindering display of gorse and deep heather at the track sides!
Best bargain was Durness, Northern Highlands. Due to strong wind, the lady said, "You won't play 18 holes" and charged me only for 9 (around 10 GBP), but I resisted the breeze ;)