The easiest/cheapest way is to buy the domain with a company that allows you full access to the domain's DNS settings and then use a service like google apps standard edition to run your email through.
Once you sign-up for the service, google walk you through the changes, and there is plenty of support available here or on google groups.
The only cost in this case would be the domain so $6 - $8 USD.
The http end can just be parked or ignored.
To answer the comment below regarding DNS
It is required for the ISP to be able to modify DNS, but how much control YOU have varies from provider to provider, so check what kind of DNS records you're allowed to change YOURSELF before buying.
In this specific case you'd need to be able to change your domain's MX records at a minimum, and possibly the CNAME and A records depending on which options you decide to setup.