I can add a keypad and 4x7seg LED display to a device for a under 5 bucks per unit, and a few days of work. Bluetooth module will cost you substantially more, and good luck making a decently looking Android + iPhone apps in that time.
A Bluetooth module costs 2 bucks, in single (!) quantity, and that's just looking on ebay. Search for CC2540, CC2541 for example.
Regarding the development costs -- if you are doing this for money, selling a small quantity of a cheap device won't cut it in any case. If you are shooting for larger quantity, it might be worthwhile to invest the extra development time, come up with a nicer interface, and sell more units?
Not to mention the impact on user interface. Forcing UI through the phone means you have to have the phone, and have an app running.
That is the more valid concern in my book, and I mentioned it in my post when I first brought up this option. A smartphone interface is not the right choice if it is needed for
every use of a
simple device. (It will make more sense if it is needed for configuration only, or for a more complex gadget which requires a more involved user interface anyway.)