I doubt Oxford make much of a profit from the tests, if anything it may be a slight loss leader but worth it to gain customers ultimately. They have to pay the company who owns the computer tests plus the staff who administer them. Surely it's good to give as many suitable applicants a chance as possible? I believe the pass rate is around 50% but that doesn't mean you make the cut for the next round as only the best X % will go through. I believe if you pass but don't make the cut you may be offered a place as a self sponsored student - provided you can raise the finance of course.
As someone says, £250 is nothing compared to the cost of the course and if you do ultimately succeed then Oxford provide the skills net based on the results of the skills assessment.