Sigh.
You all missed it. Let me point it out for you.
+1 splitting arrow splits into two +1 arrows
The +1 SPLITTING arrow becomes two +1 Arrows.
It does not say the +1 Splitting arrow becomes two +1 Splitting arrows. When it splits, it loses it's splitting properties. You can have a monk catch them, and the monk then then shoot them or sell them or give them back to you. However, when you shoot one of those two arrows, they do not split any more.
As for making them out of platinum...
I'm afraid that magic of Faerun (3.0)is quite clear in that only weapons can be made out of platinum. Ammo is not listed. If you do wish to allow platinum arrows, then you would use the value of +2,500 gp for a weapon that does 1d6, then divide it by 50. That means your arrow is worth 50 gp. In exchange you give up a +3 bonus which means your arrow, which was worth 5,808 gp, just turned into two 97 gp arrows, for a net loss of 5,614 gp.
Just call me...
THE WET BLANKET!!!