In Buddhism at the end of a precious teaching like this it is customary to offer Dana, a financial gift to the teachers, to express our gratitude and make an auspicious connection.
This is entirely optional and can be a small token. Our gratitude is not measured by our ability to give, it is the intention that counts.
We are giving Lama Alan and Dr Eva a fee, of course, so your offering is extra. Teachers never know or want to know how much individuals give.