This fails with $p cannot be resolved to a variable:

rule R when
  Cheese($p : price)
then
  int b = (int) ($p * 1_000.0); // FAILS, because $p cannot be resolved ?!
end

That error message is nonsense. It should just work. If it doesn't due to ECJ, it should say something like "cannot be cast".

This works in java:

        int a = 42;
        int b = ((int) (a * 1_000.0));

Removing the underscore fixes it:

rule R when
  Cheese($p : price)
then
  int b = (int) ($p * 1000.0); // SUCCESS
end

Regardless of an upstream ECJ problem (which should be reported if that's indeed the problem), the error message is wrong and send me on a wild goose hunt.