Im afraid it doesnt look that simple to me. The door might be re-shaped, but the rear arch is a double-skinned panel, so beating the panel isnt really an option.
At first glance, all you see is the damage next to the filler cap; that can be easily repaired. The damage to the front of the panel (next to the door) is less easy to shape properly as theres a big pillar under it and I reckon re-shaping the bit above the window will not be possible. The door might come back into shape from the inside with a beating, but itll still need shaping and filling.
I think my car had similar damage when it was stolen and recovered a few years ago. The bill then was £600 for a new rear panel and re-shaping of the door. Id allow for inflation when making a guess about current prices.