Is this really a case of an interaction? I think this can all be explained with main effects. For example, say the low price has a utility of 1.3 and at the high price has a utility of -2.6. This doesn't suggest that the value of a feature (say fuel efficiency) is more or less important. What it does suggest, however, is that to make the more expensive car as appealing as the cheaper one, we might need to have greater fuel efficiency. So instead of 21 MPG (which has a lower utility) we might need the higher-utility level of 42 MPG). And we might need to see some improvements in safety rating (higher-utility 4 stars rather than lower utility 2 stars). And so on. None of this need involve any interactions at all - main effects could do all the work.
I'm not suggesting that there will be no interactions, but I am suggesting that there is no necessity that you will see interactions or that you should expect to see significant interactions. They are not needed to perform the function of improving the appeal of an expensive vehicle.