Performance of pl/pgsql functions?

Do these tend to perform well? I have some simple formulas in functions
like so:

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...

--
Wells Oliver
wellsoliver(at)gmail(dot)com