Just noticed this thread...
To answer your question, there is no additional value to this piece. Just like for USA coinage, Panama used to make their "dimes" (1/10 Balboa), "quarters" (cuarto Balboa) and "halves" (medio Balboa) from coin silver (90% silver content). Like the US, they discontinued the use of coin silver in the '60s. The 1966 cuarto Balboa is copper-nickel, just like the corresponding USA quarter dollar.
Panama did make silver-clad medio Balboa coins through 1972, similar to the 40%-silver USA half dollars of 1965-1970.