Hate to say it, but IT DEPENDS a lot! If zoning permits you to build another detached structure, you should definitely do that. You can certainly do slab on grade, but the basement space doesn't count towards your livable sq ft if you don't have a bathroom or bedroom. It could be a media room, exercise area, kids play area and storage, which are all valuable. If you need to connect to the existing building this will have significant structural implications for your current building. If you are attaching to your existing building, your new basement can not be deeper than the seasonal high water table, which may affect your ceiling height (doesn't apply to new construction with certain requirements). You are going to want to know the water table height regardless if digging. You're also probably going to want a geotech engineer to look at your soil to determine if any remediation will be needed to build a new foundation...
Is 600 sq. ft. the footprint of the building or total sq. ft. that you can add. If it is, it sounds like you could build a roughly 1800 sq. ft addition, correct? Assuming 600 sq. ft. is the footprint your basement would roughly cost $25-50K to complete which consists of at a minimum digging the hole, creating the proper draining and pouring the foundation.
Interested to hear if anyone else can fact check me or give their experiences.
Good luck