In the condition you describe, that Old Army is very likely worth $500.
Fixed sight models tend to command a premium.
(Less of them made vs. adjustable sight models.)
Ignoring practicality, fixed sights just look "right" on a C&B revolver!
Fixed sight models routinely came in both 5.5" barrel
or 7.5" barrel configurations. The 5.5" barrel
tends to bring more. (Again, supply/demand.)
The later Old Army revolvers (toward the end of their run) have the "Warning" statement rollmarked on the bottom of the barrel. TO ME, this makes a revolver a little more desirable.
Online auctions aren't the only place to get current pricing… nor necessarily the best… but they're readily available, easily searched, and current. (Make sure you're looking at listings that actually
SOLD, though!)
A GunBroker search for Completed Items shows a number of fixed-sight Old Armies. It also backs up arfmel's suggestion. that it's "potentially a pretty good deal."
Finally…
if the holster were an OEM Ruger "Practical Holster," that could potentially boost the value by $100 (give or take).