Step one
Place an inventory item manager, set it to this:
Place a property, set it to this:
Place another property, set it to this:
(The default value is the cost of your item)
Place a button and a checker, settings:
(Button can do anything.)
Checker:
(Value can be anything.)
Place a popup, make it open and run wire pulse block when the check fails. Wires here:
That’s all for step one!
Step two, the hard part
Place a trigger that triggers on the channel “Thing”.
In the block setting, put this:
(The number block is the cost of the item.)
What this does is it finds the difference between the cost and your cash, and sets a property to that.
In the popup you placed in step one, do this in the block settings:
Why this works
So, when the amount of cash changes, it sends a signal to the IIM. The IIM updates the cash property. Now, when the trigger triggers, it finds the difference between your cash and the cost, and makes that a property. The popup is just a way of displaying that.