Thats a really tough question. Considering myself, I run my computer 24 hours a day, but when im not there, it doesnt use much energy. If you constantly use it all day, you use more power. you will see some increase in your electric bill, but you should not see such a large increase that you will want to toss it out. Just make sure that if you leave it on all the time like i do, when you leave it for long periods of time, you leave it only running necessary apps, and programs that require little to not processing power. (Every time you click on something, or a program has to process something, it uses more power)
To give you a real answer to your question. Depending on usage, you could see anywhere from a 2 dollar increase to a 15 dollar increase. I myself with a 1k PSU have seen about a $8 increase (I live in CA USA... so power is a bit more expensive here)
Hope that answers your question... sorry i couldnt give exact numbers.
The change to one's electricity is somewhat irrelevant given the fact that not everyone pays the same amount of money per KWH of electricity. I believe in the US the average price per KWH is around $0.11 - $0.13. There are areas which pay as low as $0.06 and other areas which charge more than NYC; which is roughly $0.21 per KWH.
Based on the above, 50w of extra power usage per hour can have a relatively small impact during the course of a year to a significant increase to one's electricity bill.