Salesforce Ideas is included in all editions, with the exception of Group Edition. 

Depending on how you are planning on using Salesforce Ideas, your community might be a mixture of several different types of users. Here are three examples.   

Example 1: Share Sales Advice and Best Practices
If your sales team is already using Salesforce CRM (Professional, Enterprise, or Unlimited), Salesforce Ideas is included. All you have to do is add the Ideas tab to their view and you're off and running. If you're interested contact your Account Executive about enrolling in the Limited Release.

Example 2: Company Wide Community to Drive Innovation
Many companies want to create communities which are accessible to all their employees, across all departments. It might be that you already have some CRM users who have access to Ideas. For all your other employees you can add Ideas Only Users which are $5 per month. This license includes the home tab, the ideas tab, reports, and dashboards.

The Ideas application is also included in all our Force.com platform licenses. For example you might want to give your users to Ideas plus the project management application. The Limited Use Platform License is one option. It is also $5 per user / month, but you are limited to 5 logins. If you had a 100 limited use licenses you'd have 100 logins a month to share across your users.

Example 3: Customer Communities to Capture Feedback and Manage Discussions
Lots of companies want to use Salesforce Ideas to create customer communities. The community might be used to capture feedback, or it might be a place for customers to post questions and form discussions.

The customer portal is priced on a per user basis. We figured that would be the easiest way for companies to estimate the size of their implementation and budget accurately. To get to a number you'll want to make sure you adjust it based on those who are likely to participate in the first year. Generally this number is much smaller than your overall customer community, but it is a number you'd hope to grow over time.

The Standard Customer Portal is $5 per user per month. You can also purchase limited use users which are $1 a user per month. The limited use licenses include 1 login per month, pooled across your users. For example you might purchase 1,000 limited use licenses. Within your community some customers will login every couple months and some customers will use it more frequently.

In addition to Ideas, the Standard Customer Portal also includes the ability to update your profile information, log cases, check on the status of cases, and search the knowledge base. It is up to you if you want to expose this great customer support functionality or hide it. It is important to note that the Standard Customer Portal does not include Web Tabs or Custom Objects. If you need this functionality you'd have to upgraded to the Custom Customer Portal.

With the Customer Portal you'll want at least 1 administrator who is running Enterprise or Unlimited Edition. These users will be able to manage users, customize things, edit ideas, and run reports. If you need more moderators you can use CRM users or Platform Users.

Additional Questions
If you have any questions please contact your Account Executive. They can provide a more thorough needs analysis and detailed pricing.

Also refer to the post: What Does Salesforce Ideas Look Like for CRM, Platform, and Portal Users?