Microsoft has been enhancing the contents of the ShowplanXML output for SQL Server over the last few releases and in SQL Server 2017 CU3, they introduced user-defined function (UDF) execution statistics into the QueryTimeStats node of the XML output. This was also back ported to SQL Server 2016 in Service Pack 2 for actual execution plans. This feature allows you to definitively know the impact of scalar UDF execution as part of the performance characteristics of a query. However, there is an interesting catch associated with using this feature; you have to collect the actual execution plan using an up to date version of SQL Server Management Studio or using SentryOne Plan Explorer, or the information will be removed from the execution plan.
This is a good improvement. Historically, user-defined function costs were hidden in SSMS, as you’d see the cost of a single execution. This made them look a lot more benign than they actually were.