Its getting harder - My LA uses aerial photography to catch you out.... They have sent my 3 letters in 2 years with images of a mobile horse field shelter on my land, every time its moved, within a month i get a letter demanding proof its mobile and meets regs............ with attached images.
You would need to look for 12+ acres really to stand even the tiniest chance going forwards into the future, and unless you could make a living on the land, still unlikely.
As for costs, depends where you are - you can get a 60 acre farm for £700k in parts of the dales and pennines (and nice areas too actually!) but pay £15,000 for an acre nearer cities and in popular areas.....
Its much of a muchness!