What are the best low cost businesses to build?

Businesses that do not require inventory.. you provide a much needed service.

Providing a service is probably the cheapest version of small business.
If you're looking for low cost; low maintenance ideas, things like gardening or window cleaning or garage clearing are reasonably cheap to start you off.

Prostitution

Selling lemonade and cookies? 🤔

selling stuff that you steal

going out and selling stuff