Sound generator: 14500Hz frequenzy (required), 90db (required), should run ~24days on battery (if reasonable)
I seem to recall that for 90dB SPL you need about 1W of power (given how terribly inefficient most speakers are). If we look at 24 days, that is about 90000 seconds. That would thus be 90000 Watt seconds. Assuming no efficiency losses in anything else, given a 4.5V battery, that would still require a whopping 20Ah. That's already getting into very large batteries (and I would be surprised if you get even close to that number in practice, the efficiency of your driving circuit could be poor too and you would need even more power). 

This also means that you don't need to look too much at the power of your oscillator (It will insignificant compared to the few Watt you will need for your actual audio output). I would suggest looking at something like a microprocessor with a crystal oscillator. It is overkill, but easiest and cheap in low volume. I think it will also be the easiest way to get a nice accurate tone. In volume production, I would be surprised if the generator circuitry costs more than a dollar/euro or two. Your main cost will be the batteries.