They're the cheapest (probably) per kilogram; but if you're willing to settle for smaller payloads, you can get to orbit for quite a lot cheaper. For example, the European Vega will get you 2.5T to LEO for under $40M. Half your budget on launch costs isn't bad.
I'm not sure if launching as a secondary decreases costs. They do have two missions planned apparently, after which they hope to start generating revenue by carrying up to 30kg of customer payload to the moon per launch.
