Have you considered a When statement instead of the nested if else?
something like:
var spClass =
When(
SpacTypeClass == 'ABT', 5000,
SpacTypeClass == 'Street', 750,
SpacTypeClass == 'Park', 750,
);
return (API * 6.51) + spClass;
Also if it is just returning "750" for Street and Park, your calc may be working but API is returning 0 for those cases. Double check that an API exists for those.