I have a G400 SG with the stock epiphone humbuckers...............i need a bit more from them as i would like to try and recreate zakk wylde's awsome tone (for screaming harmonics).

Would it be worth shelling out the 90 quid (brittish pounds) for the MXR zw-44 (zak"s sig pedal) or is there any other (cheaper) pedal that you guys would reccomend??

well the zw-44 is an overdrive and wont give you the best screaming harmonics.
what amp you using????
What are you putting all this gear through? Unless you're putting it through at least the right kind of amp you'll never get Zakk's tone. For the most part it doesn't come from the pedal anyway; he uses it to push the front end of his amp harder.
R.I.P. My Signature. Lost to us in the great Signature Massacre of 2014.

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That might be a problem then cos im running it through a slash sig pedal and a vox ad30vt (which has metal amp settings)..........
If you're looking for metal, get an EHX Metal muff, they own. Also, try fiddling about with the High Gain and Nu-Metal amp settings.