If you think she was offended and it will continue to bother you, bring it up to her but have your information on hand. Tell her you're afraid you might have upset her the other day and here is an actual price breakdown for the topper and that you never meant to make her feel bad. That way the ball is in her court as to the way she feels...you did nothing wrong but you can make sure she understands why it isn't $12. If she would like to barter some of her services for it maybe you can work out a trade.
Work up a cost estimate - print out prices from an actual order if you can for back up
Add in the cost of batting and a spool of thread.
Calculate the number of hours - even at min wage put in an amount.
Tell her you won't count wear and tear on the other machines and tools that someone else might add in as "overhead costs".