The Hill writes here that California Democratic Congressmen Brad Sherman and Howard Berman spent $11,700,000 in their campaigns against each other to represent the California 30th district. This is the most expensive California U.S. House race ever, when independent expenditures are excluded.
They were both incumbents, forced to run against each other by redistricting. The top-two system did not cause them to run against each other; redistricting did that. But without the top-two system, their race against each other would have been over in the June primary. The top-two system forced them to run against each other in June and in November as well, and thus the top-two system contributed to the record amount spent.