I also have a 1905 1PIASTRE with a color weight on the face. As upstairs said, 90% of 27 grams was 1PIASTRE, and the international market price in 1994 was 17.5 USD (at that time, it was 130 RMB). now
Fineness and weight can be seen below the side marked with face value (for example, fineness of 0.680 POIDS 5GR means silver content of 68%, 5g).
The following is the international market reference price of 1994 elephant coins, in US dollars.
10 point $3.0
20 cents, 2.50 dollars
50 cents and 6.0 dollars
1 peso
Large denominations are more expensive because of their high silver content.