AP and MS Colour is selling around $600-$620 cash n nets only. CP is selling around $680 but accept credit cards as well.
if u want cheap, then get the grey sets $550.
if u want with warranty, prepared to pay $600-620.
For a diff of $50-70, i wouldn't bother so much as flash is an electonic item. If anything happen, the repair bill will be hard to swallow. So get the warrantied sets and save all the hassle.
Anyway you are lucky that you are using a Nikon Speedlight. Canon 580EX Speedlight cost $780!