Save money, but don't want to build your own PC? Good luck with that one.
I honestly suggest putting together a computer of your own. You'll be able to save some money, while still getting great power for your PC. Use a website like https://pcpartpicker.com to find parts. That website also checks for compatibility, so it's a win-win.
I highly recommend getting a technician to build you a personal computer instead of buying one, that's what I did. (Except I researched and placed most of the components by myself & was supervised)
As for FPS, it comes down to the how optimized a game is (And of course if you have the recommend requirements) usually I get around 75 FPS on Rust with an nvidia 770 2gb GPU and a i5 4670K 3.4GHz with "Simple" graphic settings; this is due to my components and how optimized the game is.
Moving on to PERP / Garry's mod - the game itself is poorly optimized so no matter how decent your GPU or CPU is, you're still going to get around 50-60 and 30-50 FPS on PERP (and that's with a decent PC)