How do you decide how much to charge for custom work like gumpaste flowers or molds? They take a lot of time, so do you charge per flower based on your hourly rate? Do you increase your per serving price instead?

Are comparable items available for sale? I mean commercially. Base your pricing close to that.

The prices I find online are so varied that it didn't help me much. I found stargazer lilies for $0.50 on up to $8/piece.

If the custom decorations are proportional to the size of the cake (for example, decorations around the side) we will work that in to the per-serving price based on the approximate cost. So for a 100 serving cake, if we estimate the decorations will take 2 hours and $5 in ingredients, the cost including overhead is $65 ($5 + 2 * $15/hour for labor + 2 * $15/hour for commercial kitchen rent), so that would be an extra 65 cents per serving ($65/100).

However, if the decoration is something like a topper where it doesn't matter how big the cake is, we would just add a flat charge on top of the per-serving price.

Looking at how much others charge is a good sanity check to make sure your pricing makes sense in your market, but I wouldn't use it as a starting point, since other people may have very different cost structures.