That’s a good point. Anything that you acquired with tax free money has a zero cost basis, and your deduction value is the fair market value, or your cost basis, whichever is lower. Or in this case, if you paid a co-pay and insurance covered the rest, your cost basis is your co-pay.
However, in the case of personal property that will be put to a related use, you can claim the FMV even if the cost basis is lower. In this case, if you donated a used CPAP to Goodwill to be sold in their store to raise money, that is an unrelated use and your deduction value is limited to the FMV or what you actually paid, whichever is lower. But donating a CPAP to a charity that will specifically refurbish it and give it to a needy person with a medical need (uninsured, etc) you can claim the FMV even if your cost was lower.
You still have a problem determine the FMV though.
