The cost basis of the full 25,000 shares will be the amount that you paid for the lot regardless if you got some as a bonus or not.
Consider it like a 5:4 stock split. If you purchased 20,000 for $100,000 ($5/share) and the stock split 5:4, you now have 25,000 for $100,000 ($4/share).
Please feel free to post any additional details or questions in the comment section, especially if you received the bonus from your employer.
