The 1909/8 $20 is a very cool coin and you posted a pretty nice example. Like RWB said, the premium on an AU shouldn't be too much as it's not that tough of a coin to find. However, once you get to MS examples, the prices on these (as well as the regular 1909 for that matter) go up significantly. In all grades though, the 1909/8 commands a little bit of premium over the regular 1909 because it's the only overdate in the St. Gaudens $20 series.
Here's my example which I bought as a NGC AU58 and crossed it over to PCGS to include in my Registry set. I think I paid around $1300 for it at the time when a regular 1909 in the same grade was going for maybe $1100.
