Dharma teachings should always be free. The opportunity to make a donation may be communicated.
All the groups I know need to hire rooms and the teacher has to travel to the venue. A fee for this is fine IMHO.
Now, HHDL has the same issues - dharma free, premises and travel still to be funded.
In the case of events where he explicitly teaches dharma, the situation is pretty clear.
But what of the talks which are not billed as Buddhist teachings?
I know that the manager of a venue being approached to take a booking from HHDL probably has dollar signs in his eyes, but again we are faced with the dilemma that HHDL will probably express views which are 'Dharma'. So should there be a fee, other than to cover the basic costs of the events?
On another issue, HHDL will have security costs. He is a prime target for terrorist attacks, as is the Pope and other prominent religious leaders. I attended an NKT Buddhist summer festival recently and noted that all the drain covers were sealed and numbered to prevent (presumably) bomb attacks - these are difficult times.
In summation, Dharma should always be available without a fee. Donations create good karma. Fees for venues, travel and other costs are OK. But I am uncomfortable about the teachings HHDL may give which are not billed as 'Dharma', as I think his view will be that of 'Dharma' and therefore should be free.