How about the value of the toothpaste. Do you think Mr. Y got it for free? And yet he is going to pay tax assuming he sold it for $4.
So, the total loss on Mr Y must be $6+$4 = $10.
But as a previous poster said (sorry not reading through it all this morning to find it) value is based on what you are out; not what you say you are out because you put X$ value on an item. Do you think if a store made a theft claim on an item to the insurance co that that insurance is going to pay "retail" value on that item because the store would have sold it for that value so that is what they would be out? No...
The claim would be for the purchase price the store paid and since we don't know what that is any value you put on it is speculated. That could have been the promotional bakers dozen free tube of paste and he was out nothing for it or he could have paid $3.99 for it so you do not know...
And what if he lived in Oregon? No sales tax.... Given the information you have and the question at hand; it really isn't relevant. And if Y pays tax on his sales; then all of the loss isn't his because part of the sales price would be tax. It just keeps going; so what you do is quite simple.... follow the MONEY because that is what the problem calls for.
mon·e·tar·y (mn-tr, mn-)
1. Of or relating to money.
2. Of or relating to a nation's currency or coinage.
10 dollars traded
6 to X
4 to Y
so right now X is up $4 and Z is down 10
10 --> Z puts Z at even and X down $6