Your test is going to be more about the software than anything else. For one thing, accessing many files on a hard drive at the same time can be very slow because the head must move constantly so the videos could freeze randomly or something. If the software deals with that cleverly, it may pass the test. If not, you can't blame the board.
Thats not a problem because I cant access many files simultaneously without the help of extra people and computers without running into the problems of trying that from a single computer - so I simply wont. As you say, the outcome would be skewed. The tests are really there for use as a media centre and home file server at this stage.
For me the cost is something like 1200 SEK for the Zacate board and 2000 SEK for the Sandy Bridge combo. Then comes the cost of RAM, case, power supply, HDDs etc. Power consumption at idle isn't that far apart either if I understand correctly.
The cost difference varies by perspective with the total system cost as much as anything else, if your overall system cost is high £60 is a small number, if it is low £60 is a high number. Likewise if that £60 extra brings no noticable difference then it is simply wasted money, if it does, then whether it is considered "good value" depends on many perspectives - personally I would add a little more and buy an SSD for some systems, for others I would use an i3-2100.