The leasing costs of the 707s were naturally going to be high as they were 'risky' - there was a good chance of them being replaced on SW Suburban duties by a new combined fleet (as proved to be the case) so the ROSCO charges more to try and recuperate as much of their investment as possible. Had we seen a larger 707 order to replace the rest of the 45x, also financed via Angel, I'm sure the price would have become more reasonable - though perhaps not quite as cheap as the 701s by merit of being a Siemens product. As mentioned, 707s (and desiro cities more widely) weren't compatible with the bid requirements, making any arguments about leasing costs academic!