It obviously depends on what the G.E prices of what you are buying.
Yew long (u) 396 Gp
Bow String 186
= 582 to make a Yew longbow finished
Yew longbow 666 Gp
So you would make 84 gp profit(or something like that) for each bow you make
BUT
If you wait too long to sell them, the G.E could...