Does anybody have any recent experience of purchasing land in Isaan? What kind of price is reasonable for a modest sized (sorry I can't be more specific) area of land out in the sticks? Are there any websites I can go to to compare prices, or how have other members gone about checking what is a fair price?

land prices can be checked at the land office in your area, just ask your missus to go in and ask.
Has the land got electricity,water, how far is it from a main road, is it lower than the road, is it prone to flooding?
What do you plan to do with it, what size is it?
These need to be answered before a measured answer can be given.

http://www.udonrealestate.com/

They speak English too.

Ok, will start the ball rolling, Village 50km from Udon, 100ft x 500ft, road frontage, water and elec, 30,000 baht, including tree felling, root removal and raising up first 100ft x 100ft (~90 lorries of soil). Bargain!

In Si Saket province near Khun Han a standard building plot in the village is about 50000 baht