I spotted this 1906-S peso being sold in eBay this morning for $1325. The seller claims the catalog value for this coin in EF grade is $3200. This coin is seldom auctioned off in eBay so you might want to buy it now!
Due to its rarity, many counterfeits of the 1906-S peso coin exist, some made from authentic 1903-S to 1905-S coins.
The seller gives us some tips on how to tell a genuine 1906-S peso.
1) All genuine 1906-S have the straight serif "1" . But beware of a shaven curved serif 1.
2) Check the number "9". The genuine pieces have a larger and rounder knob.
3) The number "0" which the upper inner loop to the right of the 0 is straight rather than oval.
4) Check the number "6" which the knob should be slightly in a teardrop shape rather than round.