I would disapprove if I thought this was going to lead in any way to a rash of Bhikkhu auctions, but Ajahn Brahm is sufficiently unique in his methods that I can't really see this becoming a larger issue.
From what I understand of Vinaya, the lottery structure renders this appropriate, if a little close to the edge; if it were a direct payment for teaching, that would be considered commerce, but having a donation enter one into a raffle, I believe, puts enough space between the teaching and the donation. Just my opinion.
Gain and loss, status and disgrace,
censure and praise, pleasure and pain:
these conditions among human beings are inconstant,
impermanent, subject to change.
Knowing this, the wise person, mindful,
ponders these changing conditions.
Desirable things don’t charm the mind,
undesirable ones bring no resistance.
His welcoming and rebelling are scattered,
gone to their end,
do not exist.
- Lokavipatti SuttaStuff I write about things.