Of course it is pure conjuncture.  But:

• Statistically, most likely the step-up in significantly more than $20,000, which means significantly more tax savings.  Which is why I said it is "likely" that amending would save money.
• "Small amount of State tax"?  In my State that could be 7.85%.  Not exactly small in my opinion.
• Just because the full-year returns would be amended, does not mean it would pay to amend the partial year return.  You don't need to do the partial year if it isn't worth it.
• The OP said the client "now" told him no step-up was taken, which I infer means the OP prepared one or more of the tax returns.  In which case, YES, my fee to amend a return that I already prepared would be fairly low.  The time it takes to change TWO numbers in the software (basis and land) and to print the returns for mailing is minimal.