I have to preface by stating I don't normally sell customs. I have sold some in the past because, as Stealth Viper put it, "Everything has a price, the
right price."
But my forumla is WAY different.
First, I never consider starting cost unless for someone reason I want to remake that custom and need to get those supplies again. Sure, you probably aren't going to sell a custom action figure for $2 because it will cost $10 to get a figure at retail right now. And okay, if you spent $30 on a vehicle or $50 on an exclusive figure to use in a custom you might want to use that as a starting point. But for the most part the final product justifies the price way more than what I start with. Chances are if it is in my fodder box, I'm not valuing it very highly to begin with.
Second, and this is really where not selling my customs comes in, I put more value on the
buyer's Sentimental Attachment, than on my own. That sounds horrible, but as a non-seller, chances are I'll flat out reject a passing offer with "Thank you, but it isn't for sale." But then someone will come along at a show and be all like, "Oh my! I love so and so. I can't believe you made him. Cool, you used such and such parts on him. Is he for sale? Please say yes!" That kind of enthusiasm is hard to say "No" to, especially face to face. If their Sentimental Attachment outweighs my Sentimental Attachment chances are pretty good they're going home with it. When that happens Value all comes down to Labor.
Why Labor? Because hard customs are hard to make. One day I'll be blind and have arthritis and won't be able to customize anymore because I spent so much of my adult life toiling away giving buckles and straps straight lines and inhaling plastic, paint, and resin fumes, as well as typing run-on sentences.
For me the equation would be as follows:
(Labor + Cost if > $10) = Buyer's Attachment