deposit is not based on selling price but on the item value (if i understand the patch notes correctly).
From reading the patch notes I would say that you are correct. I will check this when I return from work and make sure that it is as we read it to be. I think the only variable which could affect the deposit would be the size of stack.
Eg. If 1 item is 100 as deposit,
10 items in 1 stack could be: 100x10 = 1000 as deposit.
I do think that the GTN takings though are 13% on the total earned through sale.
So as said above:
Total sales(including deposit return) - tax = Return
Then for those who want to know how much they made:
Return - expenses(cost of item) = profit made
(Cost of item) - total cost incurred to buy the items from the GTN, total cost for sending companions off on gathering missions etc. Obviously, comparing like or like (buy 10 items from GTN then sell 10 items) will give you the most accurate value. I'm sure this is bread and butter for most but if it helps then I'm all