It's not necessary to go into the Frankfurt Hbf to go to Strassbourg. Whenever you want to know details of German train travel, go to http://bahn.hafas.de (which takes you to a page with way too long a URL to remember). The page will be in German so click on "English".

The one way fare, second class, no discounts, is EUR 45. And that's mostly on ICEs.

Happy travels, y'all...

Dave Tucson AZ