I think that it's great result, especially because denoise.
In 20 minutes/frames, with this CPU, it's very good.
Your scene is not so easy to calculate, you have a lot of 3d DOF and complex GI.
Yes, we can see some artifacts, but they don't disturb too much.
NICE!
For the Thumbnail, just add an attachment on your thread
About bill... You told us 580 hours = 24 days.
You PC can eat 400 Watt (more or less) = 0.4 KW (you can use a Wattmeter to know exactly how much energy he eats)
0.4 KW * 580h = 232 KWh
If we assume that 1 KWh = 0.2 Euro => 232*0.2 = 46.4 Euro