How do you trademark a name .. how much does it cost?

It's necessary to research the mark comprehensively prior to filing to ensure that there is no possibility of infringing upon another party.

This entails searching the pending & registered Federal and State trademark files as well as the US National Common-Law files. Then, if clear, you can decide if you would like to file for a Federal or a State trademark.

To register a trademark, that's done either through your Secretary of State for a State trademark or the US Patent & Trademark Office for a Federal trademark.

If you are only conducting business in one state, then a State trademark is most appropriate. If you conduct (OR are planning to conduct) business in at least 2 states OR between the US & any other country, you can file for a Federal trademark.

The Federal government, USPTO, charges $325 per classification. State fees vary, from $10 to $120.

Hope that helps! I wish you much success & happiness in all your ventures!