Your error message indicates that a validation rule set on Amount Purchased is being broken.
What validation rules have you set up on this Amount Purchased?
You'll need to solve that problem before Total Cost can compute a value.
I added an image to the first post. Which should be entered. Shares purchased, purchase price, or total cost? Total Cost is the one not displaying the decimal place. Maybe I need a custom message?
Since amount purchased is used in the expression to compute total cost, you need to eliminate whatever error is triggering this validation message first.
From your screen shot, it would seem that amount purchased has one or more of the following issues:
It has something other than a number entered--could be a single non-numeric character you've entered along with the number.
The field is empty
The value in the field is not unique. -- This is your most likely culprit here. Why would the amount purchased be a unique value? This setting limits you to just one record in your table for any specific value you enter in this field.
The value is not a member of the Total Cost value lists--again this looks a bit odd as the name of the value list seems to identify a different value than what you would enter in this field
The value entered is more than 2 characters long--All you need is a two digit number plus a space and this error will trigger here if the numeric only rule doesn't trigger on it first.