Hi Luiz,
This is a public community so we welcome you to share information with your customers and us.
Currently, the Project Online Preview is free and for testing purposes only. You can keep your eye on the Official website for any updates on the price that concern you.
For the comparison between Office 365 Enterprises 3 plan and Project Professional 2013, I think they are two products who have a lot of difference. For the sales information about Office 365 products, you can see http://www.microsoft.com/en-us/office365/all-plans.aspx .
I understand you are a Microsoft Partner who works with Office 365. For Microsoft Partners, we provide two professional sites to help you get a better assistance. You would find the information you want after logging into the below sites.
Office 365 Partner Network Support
Microsoft Partner Network Support
Thanks,
Maggie Li