It's basic accounting that you consider their value. You have in your possession a set of Soulstones with a market value. They are worth a certain amount of gold. If you're sacrificing those assets to your transmutation, you're getting rid of x gold worth of Soulstones, even if you collected them yourself and didn't purchase them.
You can't just say you did part of the transmutation for free just because no money ever left your wallet. If, one day, you earn two gold and ten Soulstones worth 50 silver each, you've earned seven gold that day, even if one part of it is money and another part of it is in materials.
I'm not saying transmuting elements isn't worth it, by the way. But considering the value of the Soulstones you collected yourself is definitely the way to go about it.
Finally somebody with a clue! I was about to hit my head on the desk when another guy who can't see value of items replied with Blade-Soul Gold
. People like that destroy the market for crafters by selling stuff under production costs.
In my experience though, I've never actually gotten much out of feeding my HM pure iron offering/blue weapons. It just didn't seem worth it compared to elements (I saved up a lot of my elements that I got form leveling, maybe that is also part of the reason). I guess I'll have to double check, but I just never saw the experience gain that big.