The light-gas gun launch + rocket hybrid is a very interesting concept. It consists of a large, but conventional light-gas gun propelling a g-hardened payload 2/3 of the way to orbital velocity, then the payload uses its own engines to accelerate the rest of the way. The scheme is advertised to cost around $400M, but we should consider that within an order of magnitude. Even at that, the opportinity to significantly lower launch costs should be embraced.