The most economical way is a cheap script or WordPress site. The most economical way doesn't always equal the best way...
If your customer is serious about being in the pay site business, tell him to go all the way and get the best of everything and give himself the best chance of success. But it depends, is he a businessman or a hobbyist?
I've been in the pay site business for 12 years as a Webmaster, pay site owner, design company owner and CMS company owner. The
Elevated X adult CMS powers thousands of pay sites ranging in size from the biggest cash program networks online to small start-ups and solo girl sites.
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AJ