A gun is worth WHATEVER the next buyer is willing to pay for it.
To get an idea what the Inland you descibed is currently selling for, check the listings at [url]www.gunsamerica.com[/url]
Inland is one of the more prevalent manufacturers, so this gun should NOT draw a premium due to its rarity as compared to a Rockola or a National Postal Meter.
How "original" is it?? Do all the numbers match?? Is it a "low wood"??? (the op rod slot is longer and lower because of the way the wood is cut?? Does it have all the "early" features??
(trigger giuard, low wood, Type 1 barrel band, etc)
What you really NEED is Scott Duff's book to see what all you need to know about the rifle.
If its a completely original, near perfect Inland with all teh early features, I'd think $600 would be about top dollar.
Good luck.