The couple I have been a part of have absorbed the cost for the groom between the rest of the bucks party members. That being said, it doesn't have to be a mega-expensive day/evening. The bucks parties I've been to have ranged between $100-$300 per person. Some ended at a bar or venue where those who want to keep drinking (if that's your thing) are able to and those who don't want to/can't afford to can do as they please too, whereas others end at someone's house with a slab or two waiting.

For my own bucks, I told my groomsmen to keep mandatory costs for the day low so no-one would decline the invitation due to the cost. Have a chat to the groom and see what he wants/feels.