My standard rate when my shop was open was $60/hr, billed in 10 minute increments, with a 10 minute minimum, plus parts. So if a job took 15 minutes, the charge would be $20, plus parts. A few things, like replacing coupler knuckles, took so little time I charged a flat $5, including parts.
Other things that might actually take longer than the charge would indicate, I also had flat rates for. An 'Inspect/Clean/Lube' on a single motor loco was $60, and included replacing the brushes, brush springs, and rewiring if cloth type insulation was falling apart. Two motor locos, like the #2343 F3, was $90 and included upgrading the original P/U wire, prone to melting insulation, with 18 gauge wire.