@Zvi_Twersky I recommend you contacting to our support team as they can have a closer look at your account and give you a tailored answer here. They can also advise what would be the best plan for your team depending on the number of members you wish to upgrade.
You are right, @Bastien_Siebman. If a user has an email address linked to the Organization, you cannot change their access to Guests. In regards to your question about pricing, you can also confirm our current prices at asana.com/pricing.