Complex sentence: He is motivated to continue his fruitless search by the promise of a large reward, recently doubled, upon the letter's safe return, and he will pay 50,000 francs to anyone who can help him.
Split into simpler sentences: He is motivated to continue his fruitless search by the promise of a large reward, recently doubled, upon the letter's safe return. He will pay 50,000 francs to anyone who can help him.