I want to use my own domain here in WordPress

No. Mapping an existing domain is not done free of charge.

WordPress.COM does not accept domain transfers by mapping to an existing domain is possible. Domain mapping replaces the .wordpress.com URL with the domain URL.

re: purchasing upgrades

WordPress.com provides free blogs and hosts them free of charge. There are no bandwidth charges. All WordPress.com blogs come with 3000 megabytes (~3 GBs) of space for storing uploaded files and images. Free features are listed here https://en.wordpress.com/features/

We have 4 different plans: free, personal, premium and business. All hosting is free regardless of which plan you choose. You can view all wordpress.com plan features here https://wordpress.com/pricing/ Add a plan for each of your sites here: http://store.wordpress.com/plans/.

Details:There are no monthly payment plans and no bulk deal upgrades. We are billed annually. Also note that purchase orders are not accepted. We must pay for all upgrades in full at the time of purchase.

WordPress.com does not accept domain transfers but mapping an existing domain is possible. http://support.wordpress.com/domain-mapping/map-existing-domain/ The domain or mapping of any existing domain URL is included in the pricing for a personal upgrade, a premium and a business upgrade.

Premium upgrades do not include any premium themes which are all individually priced one time upgrades. The Business upgrade includes access to all premium themes but also note that there is no e-commerce capability under any plan here https://en.forums.wordpress.com/topic/small-business-website-1?replies=2#post-2804631.

Make sure you read the comparison very closely do you know exactly what the restrictions and limitations on WordPress.COM blogging are: http://en.support.wordpress.com/com-vs-org/

Also read Important Notes Before Upgrading http://en.support.wordpress.com/domains/#important-notes-before-upgrading

WordPress.com does not provide an email service for blogs on their sub domains or on custom domains. Please see these support docshttps://en.support.wordpress.com/add-email/https://en.support.wordpress.com/email-forwarding/

What domain mapping does is providing a seamless redirect to the very same content under the new domain URL when a person clicks the old URL .wordpress.com to the original content. It can take up to 72 hours for domain propagation to take place throughout the internet but it doesn't usually take that long. https://en.support.wordpress.com/domains/

You must be logged in as Admin http://en.support.wordpress.com/user-roles/#administrator under the exact same username account that registered the blog to access the blog's dashboard and purchase upgrades at > Dashboard> Store > My Upgradeshttps://en.support.wordpress.com/my-upgrades/

Your billing history will be at Dashboard > Store > Billing Historyhttps://en.support.wordpress.com/billing-history/

Your only options for payment are found here Payment Methods http://en.support.wordpress.com/payment/

WordPress.com provides a 30-day refund on all upgrades except Domain Registrations, Domain Renewals, and Guided Transfers. The refund period for Domain Registrations and Renewals is 48 hours.

Note: It takes from 1 - 2 weeks for the refund to be received.

You have to be logged in as Admin http://en.support.wordpress.com/user-roles/#administrator under the exact same username account that registered the blog to access the blog's dashboard, cancel any upgrades, claim a refund for any qualified upgrades, and disable auto-renew. Dashboard > Store > My Upgrades. You can also disable auto-renew there. http://en.support.wordpress.com/my-upgrades/#canceling-upgrades

Source: I want to use my own domain here in WordPress