Well this has to be really general, as I can't say what they go for everywhere. A well used, but shootable 27s around these parts goes in the $200-$300 range in .25-20. For some reason the .32-20 gets a bit more, maybe $25-$50 more.
The 1893 can really vary, based on such things as you've mentioned,; octagon-1/2 octagon. Round barrels are almost as scarce as 1/2 octagon, but most people don't appreciate them, so they don't get the premiums they deserve, unless they are lightweight round barrels.
Full octagons are the most common, and half octagons get the best premiums. Generally a half octagon barreled gun will bring $100-$200 more over an identical condition full octagon barreled gun.
Calibers can also produce large premiums, with .38-55, .32-40, and .25-36 getting the biggest demand, and in the same order. The .30-30 is a great caliber, but due to the vast number sold in that caliber, it is the least desireable. I don't let that stop me, but it is a consideration when assessing value.
I see model 1893's all over the place, price wise. I have a .30-30 carbine I bought recently for $150, and a friend just bought a similar saddlering carbine for $250. Most decent 1893's will fetch $500-$1,000, but features/condition will really affect the prices. Private parties generally sell at better values than dealers. Dealers tend to look at the internet, and base their store prices on overinflated internet auctions.
Getting long winded, so hope you'll forgive me.