If you already have NAV licenses, you should also have Microsoft Partner. If you do not know your partner and you do not have access to Customer Source, you can write an email to itmbssup@microsoft.com and they should assist you in providing the contacts. In rare cases, you might not have a partner assigned, thus customer support will assign one to you.
Then, you partner can assist you in getting the information about the prices and options of expanding your existing licenses pool. If you have older licenses, e.g. for 4.0 or 2009 version, they are not anymore supported and you might experience difficulties in upgrading.
Also note, that starting from 2013 version, new licensing has been introduced:
msdn.microsoft.com/.../jj551750(v=nav.90).aspx
msdn.microsoft.com/.../dn892088(v=nav.90).aspx
The cost depends policy for your country and the prices might vary depending on the currency and currency exchange rates, and Microsoft policy in each particular country:
community.dynamics.com/.../179576
mbs.microsoft.com/.../Microsoft_Dynamics_NAV_Perpetual_Price_List.xlsm
Also, promotions and discounts might exists, e.g. check community.dynamics.com/.../203711
In addition to perpetual model, a subscription model has been introduced. By that you can actually rent the license on a monthly basis and also vary the quantity of users from period to period, which is difficult with the perpetual license.
Finally, the partner who is selling the license, can give you additional discounts, so you should check with your partner(s).
Most of all, if you had licenses for some years but you are not aware of your partner, this most likely mean that you had not paid annual subscription fee that allows to upgrade licenses and buy additional licenses and objects.
If customer is paying BREP (annual support of licenses), he should have access to Microsoft resources, in particular - to e.g. customersource (mbs.microsoft.com/.../global).
Check more contacts here: community.dynamics.com/.../business-ready-enhancement-plan-benefits
But, if you did not pay any annual fees, Microsoft will take a penalty if you need to buy more licenses. The amount of the penalty can be that big that it might be more reasonable to skip existing licenses and buy new ones from scratch :-). Check all this with your partner.
