[Not another shrubbery]

Note that Basic already has a price structure for additional senses in a Sense-Based limitation build: -20% for one, -15% for two, and -10% for three, with more than three not being a significant limitation. To partially buy off the limitation when it involves more than one sense, you would pay the difference between the old level and the new one. You would always pay +20% to fully buy off any level of Sense-based limitation.

IMO, the book values are giving too generous a discount to the addition of extra senses in such cases, but Rev's either-or construct is too stingy. I think just halving the values (-10% for two, -5% for three) looks a bit fairer.