Basically the base Surface will offer 32GB to 64GB capacity with 2GB of RAM, and the Surface Pro will offer 64GB to 128GB capacity with 4GB of RAM.
Mind also that they would be moving from LPDDR2 RAM and eMMC storage for the Tegra 3 system and going to DDR3 and more proper SSD for the Core i5 system.
LPDDR2 shipments comprise about 70% of all DRAM as of this year, so cost per unit is likely much less than DDR3, along with consuming less power.
While eMMC (embedded Multi Media Card) is pretty much the same storage technology as used in say USB Flash drives and SD Cards. So don't provide the kind of performance as you would get with the type of SSDs used in laptops.
Along with pushing USB 3.0 support and the Core i5 alone would cost up to $200 more than a ARM or Clover Trail... So I agree, it's likely to cost quite a bit more but you'll also get quite a bit more...
The move to MCM and the 10W 22nm Haswell coming out next year though should help lower those costs. So some should maybe just wait for the second generation Surface Pro to come out.