Complex sentence: When Lilly retired in 1803, he was replaced by Samuel Ditchett, and the firm became Wills and Ditchett, which in 1804 was valued at £15,435 (2017 value: $1.67 million U.S.).
Split into simpler sentences: When Lilly retired in 1803, he was replaced by Samuel Ditchett, and the firm became Wills and Ditchett. Wills partnership with Lilly proved to be profitable, and the company in 1804 was valued at £15,435 (2017 value: $1.67 million U.S.).