Roughly a quarter of your half of the profits.

So if you bought the tickets for $300, sold for $500, made $200 to split with your friend, figure on owing about 25% of your half in taxes. So $25. Your friend also owes $25.

Probably not enough to worry about setting aside, unless the scale in the above example is badly off. The more important part is to be sure to keep the documentation showing what you paid, as only the profit is taxable.