You should be able to do without buying anything. There should be a cable that you use to hook up your monitor to your Mac and you should be able to use that same cord to go from your PC to your monitor. I've used a Mac monitor with a PC before, so it's possible.
It should look something like this:
Now I'm not 100% sure, but the PC and the Mac might have different sized plugs where this cord needs to go in. I would check first.
So just turn off your Mac and PC, unplug this cord from the back of your Mac and plug it into the back of your PC. If it doesn't fit (wrong size, etc) then you might need a little converter thing or another cord. They are pretty cheap (under $20) so that might be the only thing you need.
It should look something like this:

Now I'm not 100% sure, but the PC and the Mac might have different sized plugs where this cord needs to go in. I would check first.
So just turn off your Mac and PC, unplug this cord from the back of your Mac and plug it into the back of your PC. If it doesn't fit (wrong size, etc) then you might need a little converter thing or another cord. They are pretty cheap (under $20) so that might be the only thing you need.