Welcome to the site! Generally we focus on the "how to make food" side here, but I thought I'd suggest some things to consider from a layman's perspective. There is no standard formula for how to figure this out because so much depends on your exact situation. (The amount of meat per sandwich is the result of your own cooking style.) Be forewarned - lots of research and math is ahead of you!
You need to make a profit, so first you need to figure out how much it costs you to make the brisket sandwiches. What are your food costs (include transportation costs)? (It's important to keep your portions uniform for all units; consider using a measuring cup to dish out the meat per sandwich.) What is the cost of your time (including shopping, cooking, cleaning, advertising, and selling)? What is the cost to construct it (your charcoal / electricity, and new equipment)? How much does it cost you to sell it (napkins and other service ware, table, stall rental, advertising,)? Even if you're working "under the radar" for now, eventually you'll have to get licenses, permits and a food-legal kitchen, and you may have to change your prices then. Consider it all, and add it all up.
Once you've got a well researched amount for the costs, divide that by the number of units (sandwiches) that you produce. You need to make at least that much to break even. Probably you want to charge at least twice that (probably higher).
Analyze whether that price you've come up with will work in your target market. Do you need to increase the price to avoid leaving money on the table? Look at your competition - how much are they charging? Price is a funny thing - and you'll have to massage it a lot. (If you undercut the competition by a lot, you may just have a lowered perception of quality rather than more customers. But if you're much higher than the competition, customers may never try you.) You can also consider selling the sandwiches at not much of a profit, and hoping to make up the difference with soda pop or sides.