Agricultural land is usually very cheap.
Alas, like most thinks ... It suddenly becomes expensive when you actually want to buy it. Agric land is cheap when sold in quantity. The average price per acre quoted in publications such as "Farmers Weekly", is based on land sold around the country agriculturally, ie in large lots between farmers. Try and buy a 5 acre paddock for your pony and you'll see what I mean!
If you buy a field with the intention of using it as an airstrip it will not qualify as a 28 day rule airstrip. How can it? The field's primary use is for the airstrip itself! You will need to set yourself up as a smallholder with a agricultural holding number, and farm the land (in whatever way you choose) or rent it to a farmer, use it for horses or whatever. You can then apply the 28 day rule for your strip.
I'm sure I've told you this before in another strip related thread?
SS