A good starting point is $1000, but it really does depend on exactly what you need.
You should just tell us:
1.) your max budget
2.) pre-assembled PC? ...or can you build your own?
3.) what is needed other than the PC tower? Monitor, speakers, mouse, keyboard, Windows OS disc? All these can jack up the price.
A good price range would be at least $200 for the gfx card, at least...
You want a 23"+ Monitor, Mouse+keyboard aren't terribly relevant, and Windows OS disc will cost you $200-ish for a legit copy.
For the RAM, you want at least 4GB of RAM (You never need more).
The rest of it, you want a hard drive that has at least 7200 RPM's for speed, or bust your bank for a solid state that will blaze through everything.
Overall, I'd say $700-$1000 if you build it yourself, $1000-$2000 if you buy it pre-built.