I saw a 1905 Gillette double ring on sale for $599 on e-bay. Is this what would be called a normal price?
It's normal for a seller to try to get that price. I looked at the razor, and for that price, would expect the razor to be in pristine condition, with virtually no plating loss. Not only are the teeth on that razor missing plating, but most of the textured part of the handle is missing plating. The sellers description is incorrect by stating:
"This razor's in very nice condition with only minor finish loss".
While Double-Rings are the rarest, this one is pedestrian within that category. You can see an example of "Holy Grail" by clicking this link.
That is an example of a "Patent Applied for" double ring. In other words, the first ever production model Gillette Double ring. You can see how much better the condition is, and it sold for no more than $675.00.
At the end of the day though, any given item is worth what a seller will pay. An educated seller would pay more like $300.00 for that eBay razor in a live auction, IMHO.
Just for the record, that's the third different double ring from that seller. Trust me on this one.