That surely can't be the right general answer, because the relationship between your attitude to getting $30 and to getting $70 will depend on your wealth now. (And also on your tolerance for risk, but you might be willing to argue that risk aversion is always irrational except in so far as it derives from diminishing marginal utility.)

You could switch from dollars to utilons, but then I think you have a different problem: we don't have direct access to our utility functions, and I think the best techniques for figuring them out depend on probability, which is going to lead to trouble if probabilities are defined in terms of utilities.