I've done both commission work AND built replica props for sale.
IMHO....commission work is poison. It is some much more commitment in time, resources and energy, focusing on pleasing one certain individual......unless you have a commercial shop dedicated to it...forget it. The clients expectations are usually so incredibly high....I would not recommend it.
I've found it much more rewarding, personally and financially to MATCH what you love to build, within a market that wants to purchase that build.
I build what I love and offer it for sale when I have a few in stock. It's both enjoyable and rewarding.