Price goes up $40 per logo sold? So, after you sell 10 more logos, it'll be ($40*10) + $120(current price) = $520 per logo?
Seems like a strange pricing strategy. I guess it increases the "buy it now" urge, but at some point it will kill sales. I guess when it stabilizes you can just say "ok, that's what people are willing to pay for these". So perhaps it's a good way to explore pricing...