I would think in the region of $20-50. Apple stated a 'nominal' charge, meaning a charge which Apple deem appropriate (ideal, if you will).
January saw many of us cough up $20 for 5 new application additions. v2.0 of the iPhone OS, I believe, will see even more functionality added to these devices. I don't believe for one second we saw all of the features and applications v2.0 will include at the March 6th event, what would they do within the WWDC if so? - it'd be pretty dull if all we got for our dosh was another 'store' - in which we all ultimately generates more revenue for Apple - and some enterprise functionality which - tbh - I couldn't give a flying hoot about, being a general user - No, Apple have something
planned (Consumer Focused) to get you to dig into your pockets once more - we just don't know what 'that' is yet.
I'd guess at $34.95, they may stick at $20, but I doubt it. It's too bigger of an opportunity for them to cash in BIG. It's famed to be a HUGE update, they won't let it pass without milking it for all it's worth, after all, it's Apple

. They're seen as the designer label of desktop computing & software, the world-over.
Apple have become a software company, digital distribution and commercial software is the future of them.
Heck, they could even charge $79 and people would pay it.
R-Fly