I drive a 2002 Ford Focus with roughly 105k miles. For the last week or so my car has not had the power I’m used to. When I’m trying to accelerate it feels like it’s not really all the way in gear once I get up past about 3000 rpms - the engine revs, but it feels like nothing’s really going to the wheels. I’m worried that this is a serious problem and something expensive to fix - what might it be and what can i do?
from the sound of it, your clutch is slipping. You’ll need a new clutch, if that’s true.
I’ll second the slipping clutch diagnosis.
I also agree with the above. It need not be too expensive. Have it checked before replacing it.
Just for fun, Put the brake on tight and with nothing in front of you put it in second gear and slowly let out the clutch. If it does not stall out right away it would indicate the clutch as likely. Don’t keep trying this trick or you can cause the problem if you did not have it before. 
That’s what I was worried about - by “not too expensive” what price range are you talking about?