The figure has been revealed as a group of senior judges examined the claims of Northern Ireland's Attorney General that the anti-discrimination laws in the case directly discriminate against those who hold certain religious beliefs.
The Christian managers, Daniel and Amy McArthur, were found to have discriminated against customer Gareth Lee when they refused to process his order for a cake because of the message he requested to be put on it.
Mr Lee asked for a cake with the Sesame Street characters Bert and Ernie with the words 'Support Gay Marriage' above them.
Following a procedural court hearing on Thursday, Simon Calvert of The Christian Institute, which is supporting the McArthur family in the case, said: "The Attorney General has decided to intervene, using his constitutional power to raise questions about the validity of the legislation used against the McArthurs.
"And it is clear from the decision taken by the three judges, including the Lord Chief Justice, that he has raised matters of importance.
"These will now be fully argued when the appeal comes before the court in May.
"By the time the full hearing takes place it will be two years since the cake at the centre of what has become a legal, political and theological saga was ordered.
"So far, the case has generated legal costs on both sides which, combined, have broken the £100,000 barrier.
"The McArthur family's legal costs are well over £50,000 and I'm sure the Equality Commission's costs are at least as high."
It is understood the cost to the Christian family will be covered by donations from the public and the Christian Institute.