It is incredibly difficult to determine VAR in something related Bitcoin and it is entirely possible to design a P2P exchange without any profit to a central hub. I may assess my ROI based upon the effect having an exchange like the one BTCLuke described would have on the value of my current bitcoin holdings. For example, this exchange would functionally remove most of the liquidity issues resulting in actors like Amazon and Paypal likely entering the market. Such an increase in demand could drive the price per BTC up at least an order of magnitude.
As the initial investment is measured based upon its effect on the price of bitcoin, my investment would be proportional to my holdings of btc.
To answer your question, I would view an investment of this nature as a gamble. I would be entirely ok with losing everything for a chance at seeing this work. Thus I may consider an X-Prize style investment with a set of criterion and the first person to satisfy them would receive the prize.