Not sure what you are actually asking, but this comes to my mind by quick.
1. Is it necessary to make the XBOX version right away? Or is it possible to first release it on PC (no extra cost) and then make the XBOX version after it have shown it has potential? I do understand this wouldnt be best possible choice since piracy would have already ate lot of sales by that time, but it would be easier for investors.
2. You can show investors 3 different plans.
Plan A is to make PC version, naturally you tell them ho much money they would be missing on that one.
Plan B is to put that 1-2 million extra dollars for XBOX and get lot of money out of it potentially.
Plan C is something that includes plan B but puts on top of it a lot of stuff too, like NDS, PS3, Wii versions, and includes extra 1-3 million investment on top of plan B.
This way representing, they might start looking at plan B as the nice middle ground version.