What with today'd prices on ammo you'd best take up reloading. I don't know how many were made in the .404. I've heard 250, some sat possibly as many as 500.
I gave $700 for mine which serializes to1994. I would have considered it way too high in price but the deal included the dies and shell holder to reload, two boxes of ammo. 100 bullets to reload the 60 some odd rounds of once fired brass that came with the rifle.
I have two other Ruger #1s in the Tropical format, a .375 H&H and .416 Rigby. I haven't priced ammo for the Rigby but at the time I bought the gun it was $215.95 a box of 20. Got lucky on the Rigby as it came like the .404, with dies and components. One massive stroke of luck. Ran into a guy at a gun show who was selling factory .416 Rigby ammo for $40 a box. I bought all twelve boxes. Turns out six boxes were softs and six were solids.
Frankly, I have no idea what any of my three rifles are worth. No matter as I don't plan on selling them any time soon.
Paul B.