That would depend on your plan; one of the newer (More everything or the Verizon plan) it would be $20/month and the phone would share the unlimited minutes and texting, and have access to the data bucket if it is capable.
If you are on an even older, legacy Nationwide or similar, it would be $9.99 to add the line to share the limited minutes, and you may or may not have to pay an extra fee for texting. That is, if you can even add a line to one of those plans now - you may have to upgrade your plan to the current Verizon plan.
What phone are you considering? Is it a Verizon branded phone?