Evaluating the coverage and quality of VulDB is very easy. You are able create a free account by signing up on the web site. The main limitation of the free account is the amount of allowed access requests to the service.
This allows you to do some sanity and quality checks of the data by accessing single entries via web and API. You are also able to verify the coverage by taking a look at the following overview views:
Interested in the pricing of exploits?
See the underground prices here!