There was some discussion over the lack of plugs in the campground at our big festival (US) 3 or 4 years ago, and one of the fellows who claimed to be a know-everything-licensed-master-electrician was claiming it would cost about $100 - $150 (US) in setup and takedown for a temporary "construction pole" of the kind they put up while a new house is being built, before the lines are run in. That might not look unreasonable if you were going to run saws and sanders off it for a couple of months, but it's a bit steep for a half-day festival, unless you've got a dozen or so users sharing to kick in on the fee.

With the distance wanted here, it's likely that a couple of intermediate line support poles would be wanted, which would probably cost a bit on a per-pole basis in addition to the one with the plugs on it.

Note that as far as I know, nobody called the guys that would have to install it to check out this "expert advice;" and different markets would have different costs.

John