The profit of selling 50 rice balls a day at a stall depends on the cost and selling price. Assuming the cost of each rice ball is $2 and the selling price is $5, then the total cost for the day is $100 ($2 per rice ball x 50 rice balls) and the total revenue is $250 ($5 per rice ball x 50 rice balls). The net profit is the total revenue minus the total cost, i.e. $250 - $100 = $150. Therefore, the profit from selling 50 rice balls in a day is $150.