You have two options.
Option 1 is the most expensive option.You must become an ICANN accredited registrar which costs a non-refundable $2500 fee initially, a $4000 yearly cost (before buying any domains), a variable fee of $1,200 to $2,000 per quarter and $0.20 for each domain registered. You must have $70,000 in business assets and $500,000 worth of insurance coverage.
You must then code your own website, control panel for registration, own API's etc which would also cost easily $20,000. Legal fee's and consultancy for filling out all of ICANN's tedious paper work, why not add on another $10-20k as well.
You need to buy domains from the people who own the extensions, for example VeriSign, Inc. own the rights to ".com" which is $6.86 plus the $0.20 ICANN fee stated above, so a total of $7.06 is what each .com will cost you. And you raised a point that some registrars only charge $10 for a name, leaving them less than $3.00 (add on payment transaction fee's etc) profit per name. Other registrars such as Register.com, NetworkSolutions.com, Enom.com etc do charge $15-30+ for a domain and they seem to do good business.
More information about becoming ICANN accredited is on this page:
http://www.icann.org/en/registrars/accreditation.htmOption 2 is the least expensive and reccommended for new businessesBecome a domain reseller. Existing ICANN registrars will let you sell domains on behalf of their service under your own brand. You don't need the expensive start up costs as you don't need to become ICANN accredited.
There are many reseller websites that exist online.
http://www.resellerclub.com is the second largest (I think) reseller which allows you to start your own domain registrar within hours. The control panel and website is premade (albeit you'll need to customize it else it will look like thousands of other websites), payment processing etc is all done for you. You can sell domains from $7.69 (they're having a holiday sale) and their full pricing is here:
http://resellerclub.com/pricing/#domain_reg - I must add however their control panel sucks and the usability is plain useless for even simple things such as listing domains you own.
Another one is
http://www.wildwestdomains.com which is owned by the largest Registrar on the net GoDaddy. Not a fan of Godaddy, they are a pretty evil company.
I would advise you to utilize API's to differentiate your website from their default templates and to offer something a bit more unique else you will fail.