While electric is good for keeping a horse away from a fence (so they don't get a hoof caught in a field field or a leg trapped in high tensile), it won't keep them away from the fence if they slip into it or kick into it. I had a mare roll into a field fence with electric on top/bottom; she got her front leg caught and messed it up badly. I had another mare kick through high tensile (electrified) and she got her back leg twisted between the two wires and took the skin off to the bone.
While I realize there is no "safe fence" for horses (they can hurt themselves on anything; we had one kill itself on a wooden fence), I would certainly consider the cost versus safety factors before putting up a fence.
We have a section of pasture where we put temporary "step in" plastic post with electric tape. Cheap, easy, and relatively safe, but not something I would do as the only fence keeping them in. Your cheapest, safest fence would probably be something electric, either multiple strands of polytape or polyrope, with wooden posts. You would need to keep this tight at all times, or the wind will wreak havoc on it. Personally, I would avoid field fencing at all costs. If you use wire, I would opt for the coated wire at least; I've seen too many horses that have slid into a high tensile fence and sliced open their chest/front legs.
As far as cost for running electric: we have a very large charger than runs a couple miles of fencing (with lots of weeds), and it cost us about 72 cents a month to run. So if you are considering using electric, don't let the operating costs scare you away.