The OS-115 stem was pretty pricey when new. I want to say it retailed for about $120. But, it hasn't been made in a few years, having been replaced by the SLK stems.

It looks like they are going new for between $40 and $60 on eBay in used and new condition. I suggest you just go in to your LBS, say, "Hey, it's been eating at me that I keep riding on this stem and haven't paid you for it. I've got $50 bucks on me. Does that work for you?" Then see what he says.