Need help with estimate

I'm creating a wedding site with my sister. Her role is Web Designer, mine is developer. We already provided an estimate for phase 1 of the project, which is to create the look, elicit content from the clients for static information page, provide a contact form , Facebook link, etc.

I need help providing an estimate for phase 2 of the project, which is to make the site interactive. This involves a user management system for three types of users:
1. The Wedding Planner - Has access to vendor section, client section and pages exclusive to the wedding planner
2. Vendors
3. Clients

The functionality available in the Vendor and Client sections are as follows:

Wedding Planner Section - The wedding planner would log on and be able to view & edit a client list, a vendor list, and a list of upcoming weddings(not a calendar component). The wedding planner would also approve or deny potential vendors. This section would also allow the planner to individual and mass emails to clients with the message history being stored in a database.

Vendor Section - Potential vendors would be able to fill out a form requesting an account causing one email to be sent to the potential vendor stating your request is being reviewed and you will get a response within X number of hours. Another email would be sent informing the wedding planner that a vendor is requesting to be listed on their site. The wedding planner would be able to log on to the site and approve or deny requests. If approved, the vendor would be required to link back to the wedding planner site. The wedding planner will also be able to edit the vendor information to let people know what, if any, experience they have working with the vendor. The vendor would be able to log on, edit their personal information, and upload images.

It is a similar concept to the vendor section at http://www.chicagostyleweddings.com/chicago-wedding-prolocator.aspx, with a major difference. It appears as though the vendors listed at their site pay for their position on the list(i.e. silver, gold, platinum levels). The site I'd be developing would list vendors in the order of preference and work history with the wedding planner. Our site would also have fewer types of vendors and therefore only need to list Vendors by category(not categories and subcategories). We would also not need to create a search function

Client Section - Client would be able to log on to the site, enter their personal information, and edit a checklist of things they need to do to prepare for their wedding. This could also be edited by the wedding planner. Included on the checklist are services provided by the vendors listed on the site. The client may choose a listed vendor or enter information for their own vendor. Clients would also be able to create a guest list with and send out invitations as a mass emailing. The email would create a link where wedding guest can RSVP. Clients would also be able to upload images, creating an online Wedding album.

Additionally, all three user types would get automated reminders X number of days before the event(a default setting the user can change).User could also set up their own automated reminders by entering the subject, message, and date for the reminder to be sent. An example of this might be a reminder to pick up the Wedding dress. This is a nice feature, but if my estimate goes over their budget we could eliminate this until they can afford to add this feature at a later date.

I would be very grateful if anyone could provide a ballpark figure I could use as a starting point for my estimate. Whether it's in terms of hour or money, any information would be greatly appreciated.

Thank you in advance to anyone replying to my post.

More info

I should probably clarify that I'm not using a CMS, they are way too restrictive. I program using an MVC framework I created that uses the smarty template engine. I've already written programs that perform similar functions to what I'm looking to do. Between lifting code from old programs and using generic functions and components in my personal library, I shouldn't get stuck on any of the technical aspects.

Also, I'm only looking to get a general idea of what it would cost for phase 2 of the project(i.e. adding the additional functionality I described). We already gave an estimate of $1,875 for phase 1, which is design and static content. My sister came up with that figure because she is the designer and phase 1 is heavy on design. I'm figuring phase 2 because it requires more programming knowledge. My estimate will only be for creating the user management system and the associated web pages for the Wedding Planner, Vendors, and Clients. No need to consider web hosting, SEO, training, etc. That is all either part of phase 1 or additional features for phase 3 and up.

Thank you very much for your response. The link you gave was very helpful, I could use as much information as possible. I've worked with a partner in the past that did the estimations for me and I've been struggling with coming up with accurate estimations ever since he decided to take an office job and stop doing freelance work. Also, I told the client that I would ask a neutral party for advice and show her the responses I get as a starting point for negotiations. The client is a friend of my sister so I want to be extra careful to make sure the project turns out good with everyone feeling like they got a fair deal.