Considering the differences between the fruit fly and the mouse, the length of the investigation can be dramatically shortened for two reasons:
1. You don't have to wait years to see the results of alterations in the animal genome because the fly's lifespan is measured in weeks instead of years.
2. You can literally run hundreds of thousands of experiments in parallel because of the size and ease of maintenance of flies.
So rather than the comparison of 25 years pledge for mouse versus 15 years for the fly is should be more like 5 years only for the fly.
The other difference to point out - and this is a terrific marketing tool - is that the interactivity that can be created between the research lab, the donors and other participating and interested parties by reporting on the progress of the fly experiments on a daily or weekly basis via the web, because the experiment is over in a period of weeks rather than years.
With the experiment only needing to run for 6 - 12 months instead of years, the entire experimental investment is substantially reduced. Meaning that a prize of, say, $300k would actually be attractive to the smaller research group based on the monetary incentive alone.
The economic and timescale arguments for running a prize for fly longevity are good ones. You can get more done with less in comparison to mouse studies - even a few hundred thousand dollars would get some traction and attention within the scientific community, I suspect. You can still get a great deal of very valid science done - flies and humans are more alike than you might think when it comes to genes and biochemistry. Many of the current investigations into the genetics of aging and calorie restriction mimetics started off with fly studies.
Where the potential for a Methuselah Fly Prize falls down, I think, is on the publicity angle. I am very dubious that you could get the public as interested in flies as they are in cute, fuzzy, long-lived mice. One Yoda - with pictures - is worth a thousand press releases when it comes to attracting support for healthy life extension research.
So I suspect a Methuselah Fly Prize would do well if funded off the bat to the tune of $100,000 or $1 million - but I don't see the same fundraising methodologies used by the Methuselah Mouse Prize administrators to build funds from scratch working in this case. Not that I'm discouraging anyone from trying. If you want to launch a Methuselah Fly Prize, then I wish you the best of luck in proving me wrong about the way in which the public thinks about these things!