That's very easily solved: The minimum total price of all possible selectable options is the cheapest price that can be legally advertised. Treat non-viable options that just exist as a false baseline as false advertisement, just like it's already illegal to e.g. send out a flyer advertising TVs for $1, but there's only one available at the other end of the state/country.

It's really not a practical problem in any country that has the appropriate laws. The only thing it takes to fix this is the political will to do so.

I recently ordered cake for $x, with a mandatory selection of "size". The only possible size was "large", which cost an add-on fee of $3. Just make that type of stuff illegal yesterday. Absolutely nothing of value will be lost.