I didn't know there was such a spread in price. Is there as much of a quality difference with price as a guitar?
I'll tell you, it is much like in the guitar world. Fender and Gibson both make great banjos. There are new instruments as well as restored and second-hand versions and the price can be much different. But one must be careful because a $5,000.00 banjo is much cheaper than a used version of the same model that is an unplayable POS.
A Gibson RB-250 is around $3,500.00 (or more than $50,000.00 for a custom job like one with Earl’s name on it) On eBay, an RB will cost you perhaps a grand. If the banjo is proper and the seller is not a crook, that is a pretty good price.
That said, I can be happy with a cheap banjo like a Deering Goodtime because it is well designed and well made; it is a perfectly lovely instrument.
You can order a custom OME, Deering and other custom banjo in the really high part of the price range. And you can always order custom options from most makers, including custom fretboards or different scale lengths.
It really depends on what you want to spend. Considering the cost for an unmolested early Les Paul where prices are WAY, WAY up there, there is a great instrument out there for far less money.
You can buy the aforementioned Goodtime for three hundred or so or a long neck Deering 5-string in the "cheaper" end of a much higher price range. A $13,000.00 stock Deering is cheap compared to what sits at the top end, but very costly compared to other instruments in Deering’s line.
For example, the Deering Banjosarus weighs in at over sixty grand.
There are many smaller makers making great instruments as well. Many luthiers out there in the instrument world can build anything you want.
Finally, there are vintage instruments. Lots of rare banjos (and guitars, harps, drums, synthesizers, flutes and pianos, aplenty) out there but I will not buy one unless I can see it in person. I almost bought one a few years ago but Google tossed many warnings about the seller. Some might say thirty grand is allot to pay for a banjo and the last thing you want to end up with is a bad instrument.