You can't just blindly put out a budget like that, and not look at what you're getting.
Figure out how many GPUs you're going to want to run. The difference between 2 GPUs and 5 can mean the difference between a $50 MB, and a $150 MB.
Cpu and RAM are going to be the same price no matter the GPUs involved.
PSU is another factor, tho. Buying just a 500W PSU will work with 1 GPU. You'll spend at least $200 just on the PSUs for 5 GPUs.
For our 5 card rigs, you can pretty easily count on $200 for the MB/CPU/RAM, $200 for the PSUs, $100 for the risers, and another $100 if you're building an aluminum rig. For a 3 cards rig, maybe $150 for the MB/CPU/RAM, $150 for the PSU (singular), $60 for the risers, and $5 for a milk crate.
This is of course, assuming all new hardware (cept for the milk crate
). Buying used can save some cash.