I can't say if it common or not. I have had only two Garminfones, so I am hardly an expert. The first one was fine, more or less; the plastic sleeve holding the SIM card in place had too much play. I solved that my inserting a thin strip of plastic between the sleeve and the SIM card to increase pressure on the contacts. The second one had THE EXACT SAME PROBLEM that you are experiencing with your Garminfone. (I donated that one to Cell Phones for Soldiers.)
I just now Googled 'how to replace garminfone digitizer' and got one useful hit. A company based in Madison, WI will replace the screen and digitizer (according to them, Garmin-Asus used an assembly uniting both pieces together) for $99. They have this website:
Garminfone Cracked Glass Screen Repair | Fix Broke Shattered Screen Garminfone | Clean Planet Repair
Once the Garminfone is fixed, you choose the method of shipment back to you - USPS Priority Mail ($10), or USPS Express Mail ($29 - guaranteed overnight by 12 PM). You can add optional insurance for $10. You can add same-day rush repair service for $25.
Sorry - I know this is not very helpful. As you stated previously, it's actually more than I believe the Garminfone is now worth to do this. You will have to decide if these costs are acceptable to you.
I wish I could be of more help.