It looks you are trying to use sub-action, Add Default Value, to the action, Create new row in Model. It's clever that you are trying to use Merge Syntax to bring in the first value of the Product object, but it won't do "maths" for us, so we can't get the negative unit price by adding a default value.
I think the best approach would be to add a Formula field on the object that hosts the UnitPrice__c field. I made a formula, UnitPrice__1 * -1, that did the trick for me. Then you can reference this field when you create a default value.
Let us know if you have something different in mind.