What is considered a good ballpark figure for 3D asset work? Does one commission based on complexity, estimated time, or man hours, or agreed price?
Depends on the type of work and number of assets. It's obviously going to be based on complexity as greater complexity == greater time == more money. In my experience most artists will list an hourly wage, but you can negotiate for some sort of maximum price for an asset.
What if you are unhappy with the asset?
Make sure you won't be. Make sure that there are milestones (concepting of asset, early builds, etc.) where you can provide feedback.
How do you completely transfer ownership of the asset to the commissioning person.
Typically, these things are a Work For Hire type arrangement, where it is agreed that the person receiving the asset has intellectual control of the asset. But it is also commonplace that the artist will have the ability to use the work in their portfolio as an example of their work and be recognized as the creator of the work (while still retaining no legal rights to the intellectual property).