assuming the XML is in a file then it is exactly the same. the cost is incurred when you want to parse the data. A text file contains text, and you can read this into memory but it won't do anything on its own.
XML data can be read into memory for the same cost, but then there is a 'cost' to convert it into a parsed XML structure. this structure allows lots of things to be done much easier and cheaper, but only after converting from text.