The price is starting to go back up on bitcoins and while a lot of people are jumping out of the game there are a lot of veteran miners still buying. If you already have a 5850 I would suggest going ahead and mining! You will make more than you spend in electricity (assuming you don't pay an absurd amount). I would also highly suggest downclocking the memory and undervaluing to save a lot of electricity and heat. You'd be surprised how much it can help

. Here's and compiled undervolting results link
https://bitcointalk.org/index.php?topic=65997.0. There are also other posts about underclocking out there. If you need any of those links just ask. Best of luck!
Quite interesting..
What exactly is undervaluing? Do you only bring down the memory clock speed and voltage or also the core clock?
Thank you

EDIT:
What is a good OC tool to use? Asus smartdoctor seems to be quite limited.
Would underclocking the memory also let me achieve higher core rates?
He meant to say undervolt. Underclock mem speed, then experiment to get best ratio of high core speed to low voltage. With electricity so cheap, your biggest expense I'd guess would be component failure. Running cards hot or at a higher voltage will kill them much quicker than keeping them undervolted. That said, I had 7 5850s running for about 10 months overclocked @ stock voltage without any problems, but others seem much less lucky.
If you're running the same non-reference 5850s (ASUS DirectCU cards) I'm running, it can be difficult. MSI Afterburner is well-regarded for overclocking, but it takes some effort to get it to function correctly with those cards (I couldn't get it to work). I did use ATI Tray Tools with good success on those cards, however.
Underclocking the memory speed (which is irrelevant when mining, but you generally shouldn't clock it below 300MHz or it may become unstable) will cause the card to draw less power while operating, thus allowing you to raise the core speed (what is used in mining) without needing to increase voltage.