Cost for 1 butt weld? That's easy. Go talk with a qualified welder (experienced in this material), show him the WPS, and ask for an approximate time schedule. Make sure you provide him all the necessary info for his scope of work. E.g. does he need to make butt weld end groove prepration? Is (intermediate) PWHT required? Multiply his time by an approximate hour rate (say 60 euro's). Add to that all the other work required for the weld to be made, e.g. preheating, PWHT, cleaning, etc. which is not performed by the welder. Do the same for that as well.
All in all, you see, it depends on many things, as Dhurjati mentioned. Go out and figure.