If purchased without a plan, I'd think not a dollar more than $200US would be fair for the base WiFi model with 16GB. Make it +$25US and +$50US for the 32 and 64GB models, and +$50US for 3G support, so $200-300US total. Probably lower would be good, since as it stands, the iPad has no purpose except to squeeze money from slavering Mac fanbois... The rest of us that don't simply hate Apple right off, but have sense, will recognize that there's no reason to buy an iPad when an iPhone can do everything the iPad does, AND take pictures and make calls.
So SURE, it's bigger. But it's less-capable. It doesn't have any more RAM, nor a more potent CPU/GPU to make up for it. Just a bigger screen; not even any soft/firmware update to give it better features, like, y'know, MULTI-TASKING? (like, say, to run iTunes to play music while you browse the web?)
So in short, it is LESS than an iPhone; any extra "merits" lie ENTIRELY within screen size... And just how important is that anyway? So in short: the iPad should cost less than the iPhone. Never MIND the fact that for web-surfing and generic tasks, a netbook is better; some DO have full-width keyboards, that provide tactile feedback without consuming screen space to be open, and can MULTI-TASK. Similarly, for barebones "it's a tablet" functionality, wasn't Freescale Semiconductor demo'ing a tablet they were gonna MSRP at $100US?
This all kinda boxes in the iPad from all angles; TBH, it's why I say that it doesn't really have a market outside of fanboys. They COULD carve what market niche they could if they priced it lower; offer something, basically, halfway between Freescale's tablet, and an iPhone. But it'd need an in-between price to go with it, NOT one that's way higher than an iPhone's.