Agree with free to start, to work out the use cases and market -- unless you already have a good idea of what some of those use cases are.

For example, if this mainly targets folks doing documentation or tutorials, maybe you could add a few features or workflow things that will help those folks remove the hassle they might have in Layers (automation, preferences, constraints, ?), then charge $10 or $20 from the start.

Or maybe if used by programmers/designers to decompose/capture objects in a constrained area, you have option to save out the elements automatically as individual PSD or PNG, then sell as element extractor for $5 from the start.

Just two thoughts FWIW, good luck, seems very cool!