My opinion as a 3d artist hobbyist that hasn’t done this for a living, but I follow people and communities that do. These figures will vary and everyone has different opinions.
The simpler ones could probably be done in 8 hours, at $20 an hour MINIMUM for a decent artist, $160 per asset. The more complicated ones could take up 3 or 4 days, or $600. Obviously the artist will be able to work faster if they can reuse the same materials, techniques, and parts of other assets, so speed would go up and cost down. Also depends on the type of game and target specs, a FPS needs more detail than a top down game, I was assuming a 3rd person game like Dead Space. It would probably a month to do that set or $3000. You can ask freelance artists for a more accurate bid.
A better artist might charge closer to $40 an hour, but might be able to do it quicker.
Custom assets are a lot more expensive than packs, a pack can be sold dozens of times for a fraction of the cost of making it. And often times asset packs are either personal projects where they aren’t concerned with how much money they make per hour of effort, or they use a super highly optimized workflows where they can chug a bunch of similar assets quickly.