This is where your locker choice makes a big impact. A lunchbox locker that replaces the spider gears can often be done in your driveway without a "setup", so the cost is pretty minimal (close to parts-only). I'm in the midst of having a Powertrax No-Slip installed in the rear of my van. I'm having re-gearing done anyway, so I'm having it done at the axle shop, but I know a couple of people in my club that did the same thing at home in like 3 hours.
An E- or Air-locker, or a limited slip that affects the setup, on the other hand, gets into big labor really quick. I don't have the invoice right here, but I think my axle shop estimates 5 hours for an axle setup, plus add more if there's a lot of work to get the locker in. (ARB requires the air line installation through the case, for example.)
That said, it might be hard to justify either amount for an $1800 truck. Was a locker available from the factory? If I wasn't changing my gearing to non-readily-available ratios, I'd have just been scouring junkyards for one of the factory-equipped limited-slip axle assemblies and called it a day.