NOTE: Updated 10/29/2007 with corrections 
With the release of MOSS 2007, Microsoft and the SharePoint blogging community have spent a good chunk of time and effort touting several great features that are completely new to SharePoint. The three most prominent features are so important that they can almost be considered applications on their own, except that they have to run within a SharePoint site to be used. These three features are:
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Excel Services - allows for the presentation of Business Intelligence and raw data within a SharePoint site
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Business Data Catalog (BDC) - allows for the integration of business data from external sources into SharePoint
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InfoPath Forms Service - allows for InfoPath forms to be hosted, viewed, and created in a SharePoint site without requiring software on a local workstation
Sounds great, right? Believe me, these features are great. They can give you and your fellow users the ability to do things with data in SharePoint that otherwise isn't possible. Or at least it's not possible with the same kinds of opportunity for collaboration, integration, and security all in one central location. And they put this power in your hands, allowing you to enhance your use of business data without always having to involve your IT department every time you want to take a step forward.
But here's the thing: these features are not cheap. They're not included in the Standard license for MOSS 2007. They are only available in the Enterprise or Internet-Facing Licenses. Let me run down approximately what these licenses will cost although I'm going to warn you, it may start to get a little frustrating...
NOTE: pricing of Microsoft server software products is subject to change and/or discount. You should always work with your Microsoft sale representative to determine your actual costs.
Let's assume you have a simple MOSS environment, with a single server and your SQL databases hosted remotely on a different server (keep in mind that I am excluding SQL licensing costs from these numbers). The Standard license of MOSS for that server costs approximately $4,500. In addition to that server license, you must also buy a Client Access License (CAL) for each authenticated user who will be logging into your server for approx. $94. With the Standard License, all users must be authenticated; you can not allow anonymous users to use your site. If you decide to expand your MOSS farm with an additional server at a later date, you must purchase another server license for $4,500 but do not need to buy more CALs (I can't remember for sure, but CALs may need to be renewed on an annual basis, at their full price).
But let's assume that you've heard all sorts of great stuff about the BDC and are all fired up to use it to integrate data from your ERP system into a SharePoint site. You have two license choices that allow you to install and enable the BDC on your MOSS server: Enterprise or Internet-Facing. I'll start by covering the Enterprise license, be ready for some sticker shock. Per server, the Enterprise License costs approx. $58,000. So if you have two MOSS servers in your farm, that's $116,000 for the MOSS licenses. Additionally, there is an The Enterprise CAL adds approx. $75 to the price of the Standard CAL, for a total of about $169 per user. If you only want to host InfoPath forms for consumption via authenticated users, a license is available for about $4,400 per server and a $54 CAL per authenticated user.
<Retraction> I initially misread the Enterprise Search Server license and its entry in the MOSS Pricing link below, thinking that it was also required to enable the Enterprise feature set. For that I wholeheartedly apologize. This does change my thinking, somewhat, on MOSS's licensing situation. The Enterprise feature set is now much more affordable without that per server charge of over $50,000. Whew! But, the additional cost per user is still nothing to scoff at, especially for larger organizations. Finally, as a very weak defense of my point 
, I would suggest that my error does point out how difficult it is to comprehend the licensing options available for MOSS. I've read these things over numerous times before and never realized my mistake. Thank you to everyone who commented on the issue, I really do appreciate the assistance. </Retraction>
By comparison, the Internet-Facing license seems almost reasonable . Per server, this license will cost approx. $41,000, and the good news is that it does not require a CAL to be purchased...sort of. The thing that's not readily apparent with the Internet-Facing license is that only anonymous users do not require a CAL. If you are going to have any authenticated users with specific accounts accessing your MOSS site, you must purchase an Enterprise CAL for them. The other requirement of this license is that some portion of your site must be accessible via the Internet and open to anonymous users. So you cannot purchase the MOSS Internet-Facing license and then lock it down behind a firewall; in that case you must buy the Enterprise License. Finally, there is a license for Forms Server for Internet Sites, it costs about $22,000 per server and specifically geared towards hosting InfoPath Forms for consumption via the Internet by anonymous.
I'm going to include some links regarding this topic below that I found to be useful sources, I highly encourage you to check them out for more information about how MOSS licensing. If you note any discrepancies in this post, please do not hesitate to let me know. This post reflects my best understanding of what has become a very difficult aspect of using MOSS.
Links: