David, there is no way to just give a number. Partially because this is not a type of "kitless" pen that is usually made. The Lamy Pico, your inspiration, is $75.
How much I price my work is very dependent on the value of the materials and myself.
First I add all the costs of all parts that go into the pen. This includes everything from materials, refill, screws or other hardware, box or sleeve, stickers I include in the package, everything.
Then I pay myself. I personally add $30 per hour.
Then I add another 20-25% for my businesses profit.
Example
Materials $18
2.5 hours time $75
$93x 25%. $23
Total= $116
For me, this is the minimum. If the market can bare more, then you can ask more, but you shouldn't ask less.
Hope this helps a bit.