The CRA defines “Eligible amount of gift” as follows:

This is the amount by which the fair market value of the gifted property exceeds the amount of any advantage received or receivable as a result of the gift. This is the amount for which a qualified donee can issue a receipt.

For example, a donation of $1,000 is made to the Anytown Ballet Company, which is a registered charity. In gratitude, the charity gives the donor three tickets to a show that are valued at a total of $150. The donor is considered to have received an advantage of $150. Therefore, the eligible amount of the gift is $850 ($1,000 – $150).

https://www.canada.ca/en/revenue-agency/services/charities-giving/charities/charities-giving-glossary.html