No reason to reinvent the wheel here. Treat it as you would any other wedding reception where the client requests a specialty menu. But, beware. I have encountered quite a few brides and grooms-to-be who choose a pig roast thinking it will be a cheap and fun way to have their reception. Its fun but, it isn't cheap. Because of the time involved, its can easily just as expensive as a traditionally catered reception.
For a 100 person event, off the top of my head, it will probably cost between $20 to $35 per plate or more depending on whether or not you're offering the full treatment - place settings, tables, chairs, beverages, wait staff etc. and retain a modest profit margin for yourself.
You may want to broach the subject of cost with them sooner rather than later so they don't have any misunderstandings about what they're asking for and how much it will cost. That way, if they decide its too expensive, you won't potentially risk losing their business.
Good luck!