That's correct. Once you've built your listings through BrightLocal they are yours to keep.
The only instances you might be required to pay additional fees are:
If you need to make changes to your business information
See 'What happens if I need to change details about a location'.
To renew Data Aggregators/Networks
Whilst it's not compulsory to do so, we do advise renewing Data Aggregators/Networks each year ($100/year for all five). If you choose not to, they go into an 'unmanaged' state, which means they could be susceptible to inaccurate data about your company being distributed throughout their networks.