Becky Hansmeyer posted a couple of items recently on app pricing (and a followup) which I thought fit in with my thoughts on pricing pretty closely.

She also goes into some stuff I didn't know (but I guess makes sense in retrospect) about the non-restorable nature of repeatable IAPs, meaning if you use a tip jar method for unlocking content, if you install the app at a later date, you would need to pay again to regain the benefits.

(Also check out her adorable Corgi :)