What is the cost of having many columns as included columns in a nonclustred index?
The more columns you include to index the larger this index becomes. It affects all operations with this index (inserts, updates, selects). It takes more space in buffer pool and when you use index more data needs to be processed (including maintenance tasks like integrity checks and backups). Also when you update column included to this index SQL Server needs to update this index too (it means more reads, writes and even more locks).