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I have a Falcon that I am considering converting over to a Wave Serpent - as I only have 2 Wave Serpents and I thought a 3rd would be in order - but I don't know how many of the Wave Serpent weapon sprues I would need to order from GW to make one. 

It appears that I would need 2 of the Weapons Sprues and 1 of the Wave Serpent body sprues, but I'm not sure this is correct.

It appears that it would cost me $18 US to convert my Falcon over to the Wave Serpent (assuming I'm right nd I need 2 wave serpent weapon sprues)... I almost wonder if I should just buy a whole Wave Serpent for $20 more...

Anyone fill me in on this?  Have any advice?

Just buy a whole serpent. You will need two weapons sprues otherwise. I specifically bought just the weapons sprues so I could swap weapons out on all of my vypers and falcons.
I think it would be a better value, in the long run, to just buy a whole serpent and have a serpent and a falcon with interchangeable weapons.