It’s better expressed in terms of COG rather than wholesale price- we don’t know your margins, and it’s irrelevant in this context anyway.
The cost depends a lot on whether this is retail packaging (4 color, glossy, etc.), or just shipping packaging. Stating it as a percentage depends so much on what’s going inside, and how many you’re making. I can tell you our packaging cost is about 10% of our total product cost (this is for low volume, mid-high priced furniture). That’s for a high quality carton, with 100% paper based cushioning materials. But no custom printing, and it’s not something you would put on a retail shelf.
If you just need a wild ass guess, 10% is probably a good starting point.