SQLite Forum

System.Data.SQLite.Core SUM operation always returns as Int64
SQLite itself has no say in that, but Joe Mistachkin who maintains the System.Data.SQLite wrapper is on here too and may have some advice.

You may also get good results asking on a C# forum.

That said, as a matter of professional curiosity, what does your C# return when you do:

var sqlCmd = new SQLiteCommand("SELECT SUM(Cost * Quantity) * 1.0 FROM Parts", connection);