My fence posts cost £2 each for 3x3 square and about 6ft long, the way I was taught to put them in was to lay them end to end to give the spacing (approx 6 ft between posts)so for 100m of fencing you'd need 50 posts = £100
Strainer posts vary in price depending on size and depending on what is going in the field, the amount you will need will vary.
Stock fencing - the stuff I get comes in 50m rolls, at a cost of approx £30 for roll, so 2 rolls = £60
Barb wire comes in rolls of 200m, at a cost of approx £30 this would give you a top and bottom strand (depending on what you're putting in the field) for 100m
So around £190 plus the cost of your strainer posts per 100m of fencing would be a rough guide.
I have no idea what contractors would charge though and I'd make sure you're sitting down when they tell you
HTH
Karen