This site was recently completed by Bondi Websites for a Symposium in November. With just 5 months, we needed to make sure the client got a great site, with a perfect registration and payment system at a low cost.
The trick here is getting a monthly hosting and easy setup platform.
There is no reason to do everything custom. A template will do fine for such a site, and can be easily updated with your branding and colours. Another option is to recreate your current site, or simply add a few pages to that site.
The registration form is tricker - since you are collecting your attendees information, you need to have this hosted separately and securely. A complex form can also include IF THEN functions; price differentiation for different attendees for example.
This site was put together for $600 including 5-months hosting...probably not the most expensive part of putting on a conference but one of the most important!