Hi a GT100A is not so good:-
GT100A - short reach spark plugs 1/2"
GT100B - long reach spark plugs 3/4"
Not sure you can get race plugs in short reach check previous ngk listing Mike McD posted...
the other thing is it a blank casting? Does it suit 52/55/60mm stud spacing does it suit 52/54/56 (125cc) 61.5/62/64/66 (175cc) or come to that a 57bore 150cc ..... and has it got a squish band or not? So many varibles.
Fair price?? £5 for a paperweight, £20/£50 if it suits your requirements....