I don't know where you are but I would assume you are talking US dollars....
I just put together an MD with Asus M2NPV-VM MB - this has onboard Nvidia 6150. I just added generic RAM (1GB - they didn't have a generic 512), Cheapest AMD CPU they had in the store (which happened to be a 4000+ dual core), a DVD drive (which happened to be a writer) and a Tsunami case.
This cost me just over $300 Australian which I am sure you could do much cheaper in the US.
You could also shop around and get less RAM (only 512 is used in an MD), a lower spec CPU, DVD-ROM only. This would bring the price down and allow you to purchase other bits which you MAY deem necessary like Bluetooth Adapter, Remote Control, Wireless KB/Mouse, Better (Quieter) case.
The MB supports everything you need - except I think you need an addon board for teh SPDIF out.
The Cool & Quiet bit is where it tends to cost more. My Case & Pwr Supply was $49 and is reasonably quiet and small (although it is full height). I am guessing you could easily spend $300 on Case + Pwr to get something better.