I dont think it would always cost that much. I mean Fantasy Island repainted the Millennium coaster, but they only done that when the park was closed, they just used a cherry picker, their maintenance team, and most probably some tins of paint.
If they're already paying the maintenance staff to be there, I wouldn't really add their wages to the cost of repainting it. So that leaves the cost of the cherry picker (which they probably own) and the paint.
And I'd guess with how long it took Drayton to repaint Shockwave, that they probably done it a similar way, just with a very small paint brush.