Frankensteins are very difficult to determine price on. Namor- You and I both know what goes into these things... But sometimes it's hard for someone to understand or appreciate that value unless they've done or attempted a similar project, and know what they involve.
Only thing I can suggest is to pull together a list of mods/work/upgrades done on the watch. Total it all up with the cost of the watch included in that figure. Based on that price, figure out what you think is acceptable for such a piece, and determine two figures- What you'd ideally want for the watch (high end of the scale), and what you'd actually accept as the lowest price (low end of the scale). Perhaps you can base your pricing as somewhere in between those two high/low figures. Unfortunately, there really isn't any really easy way to determine value... So, that's why I consider the tangible items, as well as overall condition, etc. and factor in depreciation. However, if the watch is not an easy one to build due to technical difficulty, hard to find parts, etc. I also take those aspects into consideration. Short answer to a long problem... This is a very difficult value to determine.
This is why I keep most of the watches that I make. It's too much work for my brain to process