Properly map all fields (cost/amount).

lib/models/expense.dart

@@ -21,4 +21,15 @@ class Expense extends RecurringTrackedType {
             .first,
         description: json['description'],
       );
+
+  @override
+  Map<String, dynamic> toMap() {
+    return {
+      'id': id,
+      'name': name,
+      'cost': amount,
+      'frequency': frequency.title,
+      'description': description,
+    };
+  }
 }