Your's must be rare, it was the 2nd page of listings with my search set at 200 listings per page - I won't finish out the 400 mark, and this was the only one I could find which looked like yours.
I don't think you have a fortune here, but what the heck do I know?
When I left Texas in 2007, I left an old Underwood sitting in the storage building at the back of the house. I never really wondered if it was worth anything. It looked like some of the Underwood typewriters that we used in typing class in about 1957. There were a couple similar to yours, Parker4. I believe the one I left was possibly a bit older than yours. It still worked except for one key that would stick occasionally. I had used it when needed until I bought an electric in the mid 90s. One things for certain, you can always get some change from the scrap dealers! Those things weighed a 'ton'!