That is actually not a good pawn to take. Black can win it back as follows: 18...Nxe4 19.Bxe4 Bxe4 20.Rxe4 Rd1+! 21.Kh2 Rxc1 when black should be better because white has trouble developing the queenside. You should look into 16.Nbd2 or 16.d5 which lead to normal chigorin type positions. 

Also, what's with the draw offer (repetition) so early? You should be trying to beat these guys with white, not chicken out with an early draw. If you are afraid of the Zaitsev variation then I suggest you study it if you want to play the Ruy because it is quite popular, and leads to rich positions for both sides.