The standard for a USED system is generally one half of what it was totalled out to be replaced for. add the total to buy new and divide in half . You can haggle up or down depending on whose side you are own.
I do this all the time and I will not buy a used computer for over one third the cost if it has no OS and the original reinstal/install discs and the COA that goes with it. Without an OS to be allowed to check it out then you have NO IDEA as to what you are getting
For me to pay over half the cost of replacement value on a running computer It had better be in the best condition imaginable and I always run two programs from a flash drive I carry , Belarc Advisor and Everest. Both can be started from the flash drive and both gives out worlds of information. about the whole computer system..tells wether anyone is trying to pull a fast one or not.
That sounds about right. A self-built computer like that would cost about 550; without including speakers, a keyboard, a mouse, or a monitor. Then factor in the fact that it's used, and it seems reasonably priced.