Well, it depends of what you want to develop and for which API version.
If you don't need strange new features, I suggest you to develop for the base 2.1 and looking for an entry level cellphone with this OS version. I did it like that, with a HTC Magic 2.1.
If you pick up an entry level phone, and your app works well on it, you should be sure that for better cellphones your app will work fine. Instead, if you work on a brand new cellphone, too much powerful, the app could go well on your test device but it would go bad on others less powerful than yours.