I know this is going to sound dumb, but when do you know your clutch is going bad or needs some kind of servicing?

If it's starts slipping, you will get more revving and less accelleration when on the throttle hard.

I think it must be slipping.  I notice that the RPMs are way higher than the MPH.  I can be in 1st gear at 5000 rpms and I'm only going 10 mph.

How much does it cost to replace a clutch?

Try adjusting the cable slack first.  
The manual you have should show how to adjust things.
If that doesn't work, then just buy the friction plates from Suzuki.
The metal plates can be reused.
Removing the side cover and changing the plates is easy.
You can lean the bike against something and you won't need to drain the oil.
PM me if you need help.