The cheapest way of doing it Martin is to establish a repeater in your own home. It's becoming a more attractive option recently as site costs are soaring and power and IP connection are already in place. Not everybody has an ideal location though I appreciate and in some cases it is impossible to put up an external aerial. However, the nature of DMR means that even if you fill in a few dozen square kilometres, it could potentially fill in a null spot and by the very nature of its limited coverage, frequency re-usability is imminently possible.
In looking for a wide area site, it's really a matter of considering where you intend to cover, and focus on high points with a good view of that area. Farms on hilltop sites tend to be a cheaper option than existing tower and tower block sites. Farmers tend to be well disposed to earning a much needed bit of extra income, but you may need to be prepared to build a secure dry cabinet and put up a mast.