If you see the number of transistors in Xbox360 and PS3 you can see that an hypotethical PS3 without BD-ROM reader and with DVD-ROM reader could cost the same of an Xbox360 but it exist a few point that could make the PS3 more expansive than the 360:
1. The BD-ROM. I cannot believe that the BD-ROM will have a cost to manufacturate more greater or equal to 99$ and perhaps the price of the consola with the BD-ROM and with Sony losing few money will be the same of Xbox360 Premium Pack.
2. The will of wanting money from the console from the day 1 or waiting to have a good population of users is important. Microsoft is going to lose 75$ for every Xbox360 that will be sold and this is important because we don´t know if Sony wants direct money from PS3 since the day 1 or not.
3. Research and Development. Sony has put more money in research, development and new fabs than Microsoft and this will be marked in the final price.
From my point of view I expect a PS3 50$ more expansive than the Xbox360 Premium Pack.