The cheapest is to buy one copy of FM Client and develop the database on that. Then use that copy to host the file, and both remote users access the file using the free FMGo app. That arrangement will cost you one FM Client.
The other users could access using IWP and a browser, but the limitations are that your design will have to accommodate compromises as not all client features work in the browser, so you have to avoid using them or build work-arounds.
The next option is to buy two copies of Filemaker Client and both users have the same, full-fat experience on your laptop. However the person using the 'host' file will be tied to a desktop as the file must be accessible to the other guest user. Whether you find using an iPad a benefit or an irritation will dictate whether you think this is an improvement or a waste of money.
To have both users free to work remotely on laptops you would need one copy of the Client for the host, and one copy each for the remote users: 3 in total.
You could replace any one of those Client copies with the Advanced version. There is no difference in the user experience, it simply adds some tools useful to the developer, such as script debugging, and runtime creation.
Runtime versions are 'free at the point of use', as they do not need a Filemaker Client to run. But they have to be developed in Advanced, or at least the runtime created there) and cannot be hosted-and-shared.
If you are happy with accessing on iPad using FMGo and you don't want the developer tools of advanced, you can set up your whole arrangement with one copy of FM Client.
I wish FileMaker would make a demo version of Advanced available. I think that they'd sell a lot more copies of it if they did.
I recommend Advanced if you can possibly scrape up the extra dollars to do so.
The Debugger/data viewer/database design report can save you many many hours of effort as they can make both finding and fixing problems with your database much faster and easier and the debugger can be very instructive in learning how scripts actually work when you step through them in the debugger line by line with the data viewer enabled.
Then add the fact that you can customize menus and add custom functions to your solutions--both of which work when the file is opened in Regular FileMaker, but require Advanced to create and install in a given file.
Well, I've decided to go with a single copy of FileMaker Client for now. See how this whole thing works for us before committing to multiple copies. FileMaker Advanced is enticing, but since the database is now 98% completed (during trial period), I'm not sure that the extra development features are needed now. Perhaps later, if I add more features to the database. (Sure could have used that debugger in Advanced during development, though. Yea, where's the Advanced demo? Probably would be getting Advanced now had I had a chance before to use some of those extra development features.)
Thanks for the advice!
It's very unusual to create a database, put it into use and then never use it again. Very, very often feedback from your users and issues they encounter with the database will require changes to the design of the database. Often, using a new database triggers requests for new features not imaginable until users experience the convenience of using your database. Such updates and changes may never fully end for a given file. The same FileMaker Advanced features I described previously, also make such analysis and design changes much faster and easier to to.
I consider myself lucky in that, besides myself, my database has only one user -- my wife -- and all she wants is something simple to use, so I won't be loading the database with bells and whistles. For the past few years, we've been using Billings Pro for our invoicing, but we both find it to be overkill for our needs and rather difficult to use. Also, this FileMaker database is modeled on a 4D database that I wrote, modified, and used for, gee, something on the order of 20+ years? (Since the first version -- it came with a "thanks!" note signed by Guy Kawasaki, so we're talking a looong time ago.) Therefore, I knew exactly how the FileMaker version needed to work. As soon as I fix a few remaining problems, it's pretty much done. Might go back and redo a few things done out of ignorance of FileMaker's modus operandi, but no changes, major or minor, to what I've already got.
Update: After much consideration, I decided that, since the point of creating a custom invoices database is to simplify invoicing for both the wife and myself, I should not be trying to make do with a single copy of Standard and relying on FileMaker Go for the other person. Therefore, I have purchased one copy of FileMaker Pro 12 Standard and one copy of FileMaker Pro 12 Advanced. That covers all bases and provides greatest number of options for both development and usage.
Thanks for all the advice!