It is true that if you're going to buy more than 4 or so Analog Terminal Adapters (ATA-2), you'll save money by purchasing an Analog Station Module, provided you have an available fiber port to connect the ASM.
Assume the following costs:
Analog Station Module $800 or $100 per port
Fiber Station Module $480 or $30 per port
Analog Terminal Adapter $190 (per port)
The ATA-2 takes a digital port from the Fiber Station Module, resulting in a total line cost of $220 per analog port. If you need 4 ports or more, the ASM is less expensive.
The downside is that if you take a surge on the ASM lines, you can potentially lose all of your analog stations at one time. This has happened to us more than once on circuits that extend to other buildings.
The ATA-2 can be more convenient because the ATA's do not have to be located in the comm. closet with the MICS. You can unplug a digital station from the wallplate, plug in an ATA-2 and plug a 2500 set into the ATA.
These are just a few items you should consider.
Prices may vary between vendors. The prices used for this comparison are based on the Florida state government contract.