What do you consider cheap and what do you consider expensive?
The total user count that you expect on the web server doesn't give us a complete picture. There is also the nature of the solution to consider: what do the users do through IWP? How is the solution constructed?
As a rule of thumb: buy the best server that you can based on your budget.
What is your reasoning for wanting to use two servers? Will the database engine server be very busy? Maybe you can combine both budgets and buy one really good machne instead of two so-so machines?
I'm with Wim on this one, what's the budget and more importantly, what's the expected load (client-based users, activities, etc.) I'm more inclined as well to buy one good machine and manage it properly, than double the administrative requirements on boxes that may not be up to the task.
I've hosted two to three dozen active clients on FMSA11 with IWP active with up to 100 concurrent connections (on the same machine) with very few complaints over the years (mostly user issues, not hardware). Some of this was running on the older Mac minis, and now some of it continues to run under the new Mac mini servers. Fast (reasonably) internet service, 8 GB RAM, a fast drive, and a solid backup routine makes for very low maintenance solution, though the new SSD configurations are very nice and with a Thunderbolt RAID, you've got specifications that will beat just about any server that went online two years ago. The only downside I see to minis is that I'd like to put more than 8 GB of RAM onboard.
Presumably, this is for a client? If so, then $3-10K, depending on the budget. The more they spend the more options they have and the longer it'll last. But even considering that, I'd replace an production machine every three years or so, or be ready to have some down time when it goes. This is where checking your backups (restoring to another system and checking that everything is ok) is critical.
Here's FMI recommendations: http://www.filemaker.com/products/filemaker-server/server-12-specifications.html if that helps.
I'm sorry, I can see that I didn't explain fully. This is for my agency and not for a client; it's time to replace some old production servers. We have over 100 production dbs; many of these are heavily used by our staff internally but only one is needed for the web application, although there may be a couple more added later. I had discussed this with FM tech support a couple of years ago while working on another issue, and at that time they told me that the "enterprise" approach was to have a db server completely inside the firewall and a public-facing web server. That separation is what I - and our ISO - prefer in general anyway for db/web server setup (we also work with SQL Server / ASP.NET, whare the separate-server approach is standard).
As for budget ... we'll spend what it takes but since money is tight I'd prefer not to tie up funds on a machine that is far more powerful that we'll need for the next several years. We already have the db server; can't remember the specs but it's pretty powerful.