not sure if I understood completely:
You wanna set the field [tax column] depending on a value you retrieve from a different field (Season?)?
I think you could use a pick / match function combination in your script to achieve this:
pick(match([SeasonField],'RT Fare Season 1', 'RT Fare Season 2','RT Fare Season 3'),'$550','$700','$950') as [tax column]
There are also alternatives, like using a mapping table and applymap, if you got lot of seasons, this would probably better to maintain.
Hope this helps,