You're not ready yet to estimate per unit costs with any accuracy. You need to design a prototype. An Arduino Uno with shields is not a prototype. To get assembly quotes you'll need a list of components (BOM or bill or materials). You'll also need gerbers for the PCB and details about it (2 layer? 4 layer? Dimensions?).
That's the easy part. The harder part is the enclosure. Cost here is impossible to guess at without knowing anything about it.
Then there is final assembly and testing. Then packaging.
Another big expense is shipping parts and assembled units around.
And tools. You'll probably need some hardware and software tools.
How about distribution chain? Are there distributors who sell to retailers? The price gets marked up each step of the way.
All that said, its all very doable. I did it, you can too.