How much does this add to my electric bill each month?

Depends on what graphics cards you are running, and where you are located and your electric cost. On average it can add anywhere between 85-120$ per month.

How much money can I expect to make with an 8gpu rig?

That varies upon what graphics cards you have inside of your rig, for example my rig is full of non LHR 3070s and I am generating anywhere between 1100-1400$ in bitcoin per month.

What graphic cards do you recomend?

My go to and least amount of hassle cards to setup are 3070s non LHR. As well they are the most affordable card that earns the most per month.

Where can I find these graphics cards at?

I recommend searching on Facebook marketplace, offerup, stock x, and lastly ebay.