Also, people are saying wifi would jack up the price quite a bit. But look at the Zune - nice big screen and wifi at the same price point as the iPod. Once again, people are predicting multi-touch which will probably add to cost, but I don't think the inclusion of wifi necessarily means a hike in price.
It's not the wifi itself (which as you say doesn't cost too much), its the extra complexity to support things like Safari. This would also affect the speed of the CPU, the amount of onboard RAM to run the OS and any apps, etc.
From a marketing standpoint, Apple can't price a phoneless iPhone too far below the iPod, as that WOULD have a financial impact on the company. But more important is simply the cost of production and profit margin.
It's not so much the wifi but the whole bill of goods. You can swap out 8GB flash for an 80GB HD at about the same cost. So a phoneless-iPhone with 80GB HD would probably cost around $600 or so. The GSM hardware is not that expensive either, but Apple loses some downstream revenue compared to iPhone (just to make things simple, let's keep the camera in the iPod.) If we called that a wash (since Apple makes good money either way), then that's your starting point on price- $600.
If you take the camera out, you save some more. But either way you're still looking at at least $500 for the unit.
How much would a 30GB model cost? Or a 120GB model at the really high end?
So even without the wifi, this is not a cheap unit to make IMO (compared to the old iPod.) That would lend some support to the idea of a new flash-based iPod that slots between the Nano and the touchscreen model. THAT model is the successor to the 5.5G, and the iPod Touch is a brand new line at the top.
Yet another way of then looking at it, is you can stretch out the entire Apple line from Shuffle, through the various iPods, to the rumored iPod Touch, then to the iPhone, and THEN to the MacBook, MBP, etc. The price goes up, but so does functionality. They're all mobile devices after all. For example, would Apple sell more Nano's if none of these other devices existed? Yes. But as a whole, the entire line will sell far more than just 1 iPod, or 1 iPhone. Apple didn't worry about iPhone cannibalizing iPod sales, did they? And I don't think they're going to worry about the converse. What is important is appropriately filling out and planning the product line from top to bottom, not protecting the sales of any 1 particular model.
So if this is going to be a relatively expensive unit NO MATTER WHAT, I think Apple may be better off including Wifi and pricing it accordingly. Otherwise people may be put off by the price and not fully buy in to this thing, since it will seem crippled next to the iPhone (yet still cost more than other players.)