I was thinking about that the other day. If I upvote my own content, it makes me earn money.
Now lets take a concrete case. Sorry 
His account worth 22712 Steem Power right now.
When he upvote a comments, the cash prize is 21 ct.
If we look at the current price of the Steem Power we get something like that:
22 712 * 0.00563005 * 600 = 76 721€
So, with an account that worth 76721€ on the current market price, we can make 21 ct per upvote on a comment.
In those 21 ct, we have to consider the fact that 50% goes into SD and the other 50% goes into Steem Power.
In other words, only 10 ct are directly disposable.
So we will need to make about 800K comments to earn our investment back.
We have the right to write 1 comments every 20 seconds, basic maths again:
800 000*20 = 16 000 000 seconds
16 000 000/60 = 266 666 minutes
266 666/60 = 4444 hours
4444/24 = 185 days
Basically, in 185 days without sleeping (or by using a bot [Hello 
, love your work]) we could get our money back.
In 185 days, the price as the time to change.
On the other hand, if we had our Steem Power gain, we will be able to add 26 power down payment to our prize, which will lead to gain 19K€ approximately.
Not that bad isn't it?