Config:
"UseServerRewards (use server rewards as currency)": false,
"UseEconomics (use economics as currency)": false,
"UseCurrency (use custom items as currency, by specifying the CurrencyItem)": true,

Here you can use the skin id of the item you want to use as currency!
"CurrencyItem (set an item id to use as currency, default is set to scrap)": <skin-id here>,

I use Blood:
"CurrencyItem (set an item id to use as currency, default is set to scrap)": 1776460938,

Language File:
Than you need to go to:
/home/<your server>/serverfiles/oxide/lang/en/TreePlanter.json
I use Blood as currency so I changed this:
"Message.Item": "{0} | $Blood {1}",

Change the amount in the cost line of the config what you want the tree to cost.