Magento B2B is basically a feature set available to Magento Commerce users. Magento Commerce, formerly known as Magento Enterprise, is the paid version of Magento. For better or worse, Magento does not publish license rates. They are known, however, to price these licenses based upon your store's revenue. I haven't heard of anyone paying less than about $40,000 a year lately, but if you want an accurate price, the best thing to do is reach out to the Magento support sales team. If your site is small, the license may be less.
Alternatively, you can use Magento Open Source, formerly known as Magento Community edition. This is the free version of Magento that's used by the vast majority of businesses on Magento today.
You can learn more about Magento's B2B features here: https://www.nchannel.com/blog/magento-2-b2b-multi-location-inventory/
You can find Magento extensions that provide similar functionality to some of these features in the Magento Marketplace here:
https://marketplace.magento.com/
Overall, Magento Commerce's B2B features are relatively new (in the grander scheme of things), so users have historically relied on extensions and customizations to add these sorts of features.
Keep in mind that even with Magento Commerce's native B2B features, you may still need some customizations or extensions to meet the full business needs of your B2B operation.
Best of luck!
