Depends what your aiming for, if your going for extreme overclocks and so need the best temps possible then 10 bucks for a few degree's drop in temp may well be worth it.
The paste that comes with aftermarket coolers generally aren't too bad these days, something like MX-4 may shave off 5C from your load temps, then again it may not.
I just happen to have an original Coolermaster Hyper 212 in my own personal pc. Switching from the stock Coolermaster thermal compound to IC Diamond 7 Carat thermal compound resulted cpu core temperatures dropping about 4C.
Individual results are guaranteed to vary. I should mention that there are a lot of thermal compounds available and everyone has their favorite. One of the fun things about building or upgrading your own system is that you can experiment and find what works best for you.