When you say the server should send the response email, I think you are assuming a web site linked to its email server in its same domain ? Surely that would save a step and be reliable, once set up, for an automated response. However that is assuming that you are keeping the auto-increment logic there as well, along with the customer database and all related biz data. If you want the app local, on your own PC-Mac, then there might be some decisions. Same thing if you want more personalizing in the response email. If you are trying to do a small, simple app, it seems like forcing everything unto your host server adds some complication. (And may help explain the $1500 price tags above.)
And if you auto-check your mail every minute, or have some sort of push-passthru-download (including the simplicity of Gmail forwarding which does work great) I really do not see any real functional distinction. My email I estimate at better than 99.99% fine for those activities. (I pass a lot of forum mail down to boxes on my PC.) What is the advantage of a 3-second or 1-minute save ?
The point is simple. The email response itself might be excellent in the cloud. However if you decide the full app should reside on your disk, then you may not want to divide responsibilities.
Another point is that you would have fairly elegant customization if your "Filemaker" had nice hooks.
Note that these database programs will also have "server" options, but again, why spend the extra money and hassle for a KISS application ? Later, you could always port your app to the server. Even Filemaker, known for its proprietary files (e.g. .FP7) I believe now will run on some sort of SQL database as an alternative. (Check this, though, if it is important.)
The idea that you will simply hand-code in PHP the overall functionality of a Filemaker app is simply a non-starter, imho. Where I program on an iSeries, one exec actually set up duplicate functions on Filemaker just to get the really nice email, PDF, word-processing, excel-export, PC style data-base integration, running local server multi-user (including a lot of communication and documents with vendors overseas). I'm actually in the process of deciding how to best re-unify the system, while letting the few users maintain that nice simple functionality. (I'm even considering simply forcing all data entry to be on the mini and then refreshing the data periodically. At least at first, so that no functionality is sacrificed while playing with alternatives to Filemaker.)
I can see the plus for a mission-critical response. My internet connection has an outage, usually a couple of hours, every few months. If you absolutely had to have quick response all the time, that might be unacceptable, since you do add another traffic point. The other day I had to reboot my router, and was down for a bit, a quirky situation. Clearly, you would avoid the occasional local point of delay.