Budget mother of bride wanted a $1500 wedding cake and takes up to a month before wedding to say i only have $500 and 200 guests. 

I bent over backwards catering to her and made it happen. Sketches, prices, texts galore. 

One mistake was made---- I put white chocolate mousse filling instead of raspberry mousse on an 8" round and a half sheet. I didn't realize because so much was said,  I thought she had changed it. 

When she (mother of the bride) said everything was great but the couple was disappointed,  I said I'd refund the cost of mousse filling. $40 and apologized. She said they may not even want a refund, they liked the cake. It was wonderful. 

2 weeks later I get a text. "The couple didn't get what they paid for. The order was wrong, $40 isn't enough to make it right. "

It's all the mom, so I asked for the bride to call so I could make sure I understand.

How much should i refund? 

Under the circumstances, I do not think you owe a refund at all.  You went above and beyond to assure the bride had a cake of which she was proud and she liked the cake.  I think mama just wants her money back.  If they return the cake to you, then you can refund for the portion not eaten.  If no cake is returned, no refund.

That being said, I am not a professional and do not depend upon customers for my livelihood, therefore I am much less tolerant of customers complaining after the cake has been consumed.  If you feel you should make a refund for goodwill purposes, $40 for the filling is more than sufficient.

This situation is why it is important to have a contract with all the details and signed off by the client.  

I agree with both people above...

$40 is more than enough of a refund. I think she is trying to rip you off and get a free cake out of you. I would hold your ground and only refund for the filling. The wedding is over, they loved the cake.

For future orders, ESP weddings contracts are the way to go.