On trade broker, what does the total and per unit mean?

total is how much you will pay, per unit is how much each one in the stack you're buying costs

When you put an item in the broker, it asks you to enter "Price per Unit"- that is how much one single one of that item is being sold for, regardless of the stack size.
The broker automatically calculates how much a stack of whatever amount will cost from that.

So if I'm selling 'X' for 2500g each, and I put a stack of one 'X' on the broker, it will be sold for 2500g, but a stack of four 'X's in the trade broker, the total price someone will pay is 10000g to buy it

Thanks for the help!