Imari pottery used to have excellent collector value. Unfortunately, the market for it has pretty much died over the last ten years. Only the most exceptional pieces hold any real value. The fact that this is signed is not necessarily a game changer. There were hundreds of imari factories in Japan, and some date back hundreds of years. You would have to have that dish evaluated to determine its value, but it is highly unlikely that you will have found a real treasure. It's a nice piece, but not unusual by any means that I can see.
If you go to Christies auction house online, they have a free evaluation program on there by experts. It will give you an idea of its true worth.
Cheers!
(I lived in Japan for a cumulative 15 years of my life, and speak the language fluently. In addition, I am a licensed Auctioneer and have put a lot of asianware through my auctions. My family has LOTS of Imari from the time my father was in Japan during the Occupation, as well as other artwork, some of which IS in fact very valuable. I wouldn't hold my breath on this piece, but you never absolutely know.)