Chill, usually when I build these the price will go up some odd $50-100 because all the deals that were detected have ended by then because they buy it a week-month later.
I have some notes to pass on just as general information that you might find useful later:
For Win8 (if you want to get it instead of win7, there's no reason not to get win8 though.) I'd recommend getting
Start8 when you can because it helps maintain the original feel of windows user interface, if you still find metro annoying after using it for a while.
The 7850 isn't exactly one of the highest end graphics cards on the market, it'll handle most things, but don't expect it to give you the output of a $400 card or be upset that it doesn't run crysis 3 on ultra at 100 FPS.
I can't recall if PCpartpicker totals in taxes or not, I don't believe it does, but it'll most likely end up costing you in the $1200 or so if it doesn't up front, then you can get $70 back via mail in rebates.
Seriously though, when you get enough I'd look into getting a better mouse. Speaking of which, I asked you on your preference of wired/wireless because that's honestly the biggest difference in that price range. Most casual users opt for wireless because "lol convenience", though I personally can't stand having to deal with the batteries, a tiny little wireless USB dongle you'll lose somewhere, or it cutting out when the battery dies. Also the fact that wireless transmission is slightly slower, though you'd only really notice it in pretty competitive game-play.
If you absolutely HAVE to get the price down some, you could cut out the OS and install a trial of win7 for approximately a month if you still have the installation CD from your last PC laying around somewhere, or I guess you could DL an install disc from the internet as well if your current optical drive is capable of burning discs. (After the month ends you can still use it, but it'll be a bit annoying. you can then get whatever OS you need when you can afford it however, either that or deactivate one of your older machines that you don't use and use it's license on the new PC, if you happen to have any lying around.