best economical evergreen hedge to plant in aberdeenshire

The direct answere to your question, Andrea is Cupressocyparis leylandii but it comes a a great health warning! It is a rapid grower and, without good care, can become a nuisance, particularly to your neighbour. Keep it well trimmed and it will make a fine hedge.
Your big problem in Cuminstown is the wind (I bet its blowing a hooly right now!) so you might consider a thorn hedge. Not strictly evergreen but it will hold a lot of its leaves through winter and is more attractive than leylandii.
Be interesting to see what Bscott suggests.

One of the nicest I think is either Cotoneaster franchettii or Cotoneaster simondsii, both hardy and will tolerate coastal aspects well and they clip into very dense hedges of moderate height and quite quick too.