On 820K LUTs if your placement was no more efficient then Spartan-6 boards maybe 1 GH/s. Spartan-6 is kinda hard to place though so maybe you get 50% better (50K LUTS per SHA-256 loop). Routing is greatly improved so a 4ns critical path is plausible. That means running the chip at 250Mhz. Ballpark we are talking ~ 1.8GH/s. If you could achieve 3ns critical path you might get that up to 2.4GH/s. That is on the largest Stratix IV which runs ~$8K. You would have a higher cost per MH/s on the smaller chips. That was all back of napkin guestimate. Don't go buying $9K chips based on that.
The power of Stratix line comes from the ability to perfect a design on a $9K chip which can be reprogrammed at will. Once you perfect that hypothetical 2GH/s you build a half dozen and torture test them. $100K investment sounds like a lot but honestly it isn't compared to the huge fixed costs and large run sizes of sASICS. It allows your team to perfect the design, tweak out any PCB issues, bugs, glitches, power and cooling concerns, etc. Catching a problem here is relatively cheap. You can get new prototype boards with 3 day turn around for <$100. Remember the goal isn't to be economical. It is all about perfecting a design as quickly and cheaply as possible.
Once you are completely satisfied with final design and your investors sign off, Alterra would "clone" that bitstream to produce a mask for etching sASICS. The mask is going to cost you $100K+ (my number is probably dated, it might be triple that). The good news is the sASIC of the same chips is likely less than $2K each in bulk and probably uses 33% to 50% less power.
Still you are looking at a minimum order of 1,000 units and 5,000 units would be more cost effective. There is no "do over" on a sASIC. If your chips have some critical flaw which produces invalid hashes under certain conditions well you got $10M in defective chips (it happens sometimes even to Intel). With those kind of numbers I think you can see how getting it right on a "cheap" $9K chip makes sense.
Chips like Stratix are designed to allow low cost, low risk PROTOTYPING not retail use. Only if the market segment is so small that you could never sell more than a hundred units or so (like some high end enterprise networking devices) would you use the chip in a retail product.