Hi, it's relatively easy to build a membership site that you need but the knowledge and training for you are much more valuable than smaller budget.
Please let me know if you need a site only for yourself (and you will create courses) or you want to offer creating and hosting courses on your site to others (as on sites you have mentioned)? It's interesting that both stated sites are built with Drupal (content management services solution) while using WordPress would be much easier to maintain for a long time.
And finally, sites like you have stated have much more then just simple learning management software (LMS) features, typical free software is for example Moodle. They have reporting, (better) billing and other advanced modules which are usually not free etc.
I'm interested in your opinion. Realistic - but even that is heavily discounted - bid amount for developing such website (using open source LMS) was entered. For few hundred dollars I would install only basic WordPress site with some LMS features (quiz, progress indicator) and membership functionality (automatic registration after payment with PayPal or other service, upgrading levels, access policy) plus showing you how to create courses etc.