Why is an IDSL connection more expensive than a 1.5M/384K connection?
IDSL can be thought of as modern ISDN. The technology for IDSL is different than DSL, and since it is not as mass-market, it is also more expensive. Your money is not paying for bandwidth, it is paying for equipment and installation that is not as frequently performed, and therefore costs more. To some extent, also, IDSL prices maybe higher because they can be: IDSL is often the only choice if you are sufficiently far away from your CO.