Under USD$1000 excluding monitors is more realistic. And you should have plenty of choices.
You shouldn't be concerned yourself with mobo (motherboard) if you are to buy a complete box (recommended) instead of assembling your own.
Visit your local computer shop like Fry's or something. Look around. Many makes/models to chooce from. The prices are competitive. The key factors of the pricing are:
1) The processor used (from not so fast to really fast)
2) Memory installed (the higher the better)
3) Any higher-end peripherals (e.g. blu-ray DVD drive, etc.)
So it really depends on what your needs are (which you had told us nothing about).
This is like asking: hey guys what car model would you recommend to buy for under $10,000?
If you trade using an internet browser, then a low-end PC will do. Intel i3 class and equivalent AMD processors. Price is about $500 or so.
If you trade using many charts, maybe a mid-end PC. Intel i5 class and equivalent AMD processors. Price is about $800 or so.
If you need to do a lot of backtesting, real time scanning, real time charts, heavy number crunching, then maybe a high-end PC. Intel i7 class. Won't be under $1000. Probably around $1200-$1300 and up.
The other thing to look for is how many PCIe slots available for expansion. These days most pre-configured boxes give only 1. It's only enough to drive 4 monitors if you buy a 4-head display card (more expensive). Or you can buy 2 dual-head display card (PNY, EVGA, etc.) for about $50-$60 each. But you must have at least 2 PCIe slots.
