Usually these type of structures are handled by creating an alternative-specific design: https://www.sawtoothsoftware.com/help/lighthouse-studio/manual/index.html?hid_web_cbc_designs_7.html
In the example in that help page, it's kind of implied that walking or biking has no cost. If we wanted to specifically call attention that there is no cost, you could add an additional attribute with two identical levels and then exclude it during analysis like Jacob mentions. You'll have to do a little trickery because Lighthouse Studio won't let you have exactly the same labels for two levels (toss in a space character or something will probably work).
It's probably easier to set up a conditional display table, though. This will allow you to "fill in the blank" so when walk combines with missing (0 level) of price, the conditional display table could display the word "Free" instead of a blank. But it's purely just a display thing, there is no "walk price" attribute and there is no "free" level in the design, it's just text displayed during the exercise.