Megaventory is a subscription based service - you can see the costs involved here.
Magento is a solution you have to install at a server you own or rent. Although the Magento application in itself is free there are costs involved in setting the eshop, importing data to it, applying customizations, maintaining the server, etc. All these are usually covered by the fees of a software developer or software house specifically hired for these tasks.
You can also use services such as MageCloud and QuickMage which offer setting up and running a Magento eshop 'as a Service' - essentially they get paid a monthly fee to manage your site so that you don't have to hire expensive external help.
The Magento extension connecting Megaventory and Magento is free.