Having just gone through this process myself, here are the things I kicked myself about:
1) Plan it out first, make a list of what you will need - length of string, no of insulators, handles, stakes etc
2) shop around for prices - I reckon it's cheaper to go mains if possible unless you already have batteries and charger. I started with a borrowed battery energiser from a local dairy farmer and fixed a broken one he had which he let me keep. Cost of batteries was more than mains unit, so I upgraded.
I ended up paying the following:
£0.70 each plastic stakes (cosmetic seconds)
£16 box of 100 ring insulators with drill attachment (local ag merchant is £8 for 25)
£27 each for 1100m roll of polywire (local ag merchant is £25 for 500m)
£1.16 each for round wooden fence posts direct from saw mill
£14 for 1m galvanised earth stake
about £30 for lead out cable.
3) If you can afford to, buy everything you'll need at once - if I had bought big boxes of insulators and large rolls of polywire initially, rather than buying from local guy at 3x the price I could have saved myself a fortune.
4) If you're just doing the pig run, and have a mains energiser, it would be worth putting a switch on the fence, so if you need to turn it off while you're in with the pigs you don't have to run all the way back to the barn.
5) buy a tester and use it - then you'll not be awake all night wondering if you connected the fence correctly!!
Hope that helps...