I fully expect $150 - 250 more than the iPhone.
Current iPhone 3GS is $600 base. $200 after rebate. However, that's an older price point, and I expect to see it drop to $449 pre-subsidy ($149 for 16GB with a plan) with the tablet release announcement as it's mfg cost is now around the $300 price point. That would put a tablet on the street soewhere around $599-699 without a subsidy, and $249-299 with it.
Lets not forget the rumor of both an iPhone OS tablet, and a slightly larger OS X tablet following it. The iPhone OS tablet, even with a 9" screen, can't be priced too much higher than an iPhone. a 10.4" tablet with OS X and iLife, and a beefier CPU will likely be $800-949 (maybe $300-400 discounted with a cell phone plan as well). Still, not bad at all. A 13" White macbook may be a good bit more powerful, but considdering the utility of the tablet and the high cost of miniaturization (and the SDD it will likely include), plus being full multi-touch, even the same price as a 13" would not be unheard of. (especially if NFC is included and it pairs well as an extention of a real Mac).