No refund is available. The US$15,000 processing fee is charged per person and is described as nonrefundable. It is entirely separate from the gift to the U.S. government, which comes later and only after vetting has been passed. An applicant who does not clear vetting has spent the fee and has not made the gift.
Two payments, two moments, and only one of them is at risk early
The separation is deliberate and it is worth understanding before anything is paid. The fee buys consideration of the application. The gift is the substantive payment and follows a favourable vetting outcome.
Someone with a complicated history therefore risks the fee for each member of the household without reaching the stage where the larger sum is engaged. Hypothetical example: an applicant from Sainte-Cécile-de-Milton with an unresolved matter in his past pays processing fees for himself and his spouse, and vetting concludes unfavourably. The fees are gone, and the family has learned something it could have investigated first.
Where any history exists, the cheaper sequence is to have that history assessed by counsel before fees are paid at all. As with every figure in this framework, the amounts and their treatment must be re-verified against official materials before money moves.