Define fair. Anywhere you go (that I've seen) you'll pay more (quite a bit more in some cases) than you did for your revolver. That said, contact the usual suspects discussed on this forum: Bowen, Clement, Gallagher, Harton, Stroh, et al.
On Clements website it shows: Octagon barrel on SA conversion utilizing a new barrel- add $325 Note this a add + $$ to a "SA Conversion"
and on the Old Army; Rebarrel with octagon barrel- includes dovetail front sight, $600
There's your range, it is my understanding that most do just the barrel job for ~$500-600.
Freedom Arms; +$475 for a new gun and rebarrel a Existing Gun $744
This is something I was looking to myself recently, but I've decided it's not feasible for my budget.
From Bowen's website:
No. B325 (*) Octagonal Barrel Classic barrel style is suited to most Ruger single-action revolvers. Made for premium-quality barrel stock in both blue and stainless in 5 1/2 and 7 1/2 inch lengths Please specify) and is available in .44, .45, .475 and .500 (Linebaugh) calibers. Other calibers may be available by special order. Features include an external, integral recoil lug (suited only to steel ejector housings) and an integral front sight base which accommodates S&W DX hand-detachable front sight blades with a serrate ramp as standard. A wide variety of blades are available as options; inquire. Installation also requires cat. no. B250.
B325 $995
Click on the link if you want to read more or see a few pictures of his work: