If you have a look at "shareoplanes"on offer, you will find that the standing charge is as much as that hypothetical day's gliding, add hourly flying-rate +fuel or wet-rate (includes fuel) and i'd guessthe shareoplane would cost~ 4 x the cost of gliding.
As the sages have pointed out, you are highly unlikely to get a winter flight where the surcharge kicks in, so, you can safely say the maximum cost per flying-hour = 2 launch-costs if towed , alternatively, about 4 winch-launch charges.
You may spend 8 hours at the field, but the figures say you will manage less than 2 hours flying in that time.
Why not budget a month's flying-spend....maybe you'll blow it all in one day, then have to "sit-out" the rest of the month......but, more usually, if your figures are sensible, you'll get days where you spend well over the amount "allocated" and others where weather frustrates you and all the cash in the world cannot buy a change in the weather-pattern (but you could go to where it is more favourable, if money was no object.)