CS is made specifically to be able to run on low-spec systems at a decent amount of fps.
That being said, if you're planning to play it competitively on any level, you will need a pc that will run at least twice the refresh rate of your monitor (120 fps for 60hz, 240 fps for 120hz and 288 fps for 144hz).
CPU:
Any i5 or i7 from the 3rd generation onward (so i5-3xxx or higher) is sufficient for CS specifically at these framerates.
If your budget allows it, specifically the i5-4590 coming in at $179,99 and i7-4790K coming in at $299,99 are not only strong cpu's, but have a very strong price-performance ratio.
RAM:
8GB of RAM at any speed will do. I have tried CS with 4GB of RAM, it's not advisable. 8GB of RAM is, for CS, as good as 16GB or 32GB. No significant increase in performance will occur past 8GB so yeah.
8GB kits of RAM can be found everywhere for ~$30. If you can, get one with atleast ddr3-1444mhz. Other than that make sure it's compatible with your CPU and u should be good to go.
GPU:
This one is a bit tougher to explain, so instead I'll give you a link.
Any card with a score of above 4.2K will run CS:GO with sufficient amount of FPS for any monitor. Some might require tweaking down the settings, but that is no problem at all in CS where the game is focused not on the graphics but on your performance.
http://www.videocardbenchmark.net/high_end_gpus.html
Hope this helps. Assuming you have all the other necessities you should be able to get a strong CS-machine for under $400. Good luck.