Performance of pl/pgsql functions?

CREATE OR REPLACE FUNCTION public.stat_foo(a integer, b integer, c integer)

  RETURNS numeric AS

$BODY$

declare ret numeric;

begin

select (a+b) / c::numeric into ret;

return round(ret, 3);

end

$BODY$

LANGUAGE plpgsql IMMUTABLE COST 100;

The reason I'm doing this is because i repeat this formula in a bunch of views and queries, and it's easier to have one function. Would this somehow be slower than reproducing the formula in every view its used? I'm hoping not...