Maybe it was so cheap because it doesn't seem to do much
Although I do not share your view on this mission and on why its payload is limited, I believe you correctly identified the trick to limit costs: Keep and simple.
Management costs are not linear with mission complexity. As the payload and complexity increase, so does the risk of something going wrong, leading to increased costs in planing and designing the whole thing. Because the costs are higher, the pressure for success increases and the need to cross-check every detail arises, implicating even more costs. You fall in a upward spiral for costs. Interfacing/integration costs are of course also higher with more complex mission, but they are not as non-linear as management costs.
In keeping a mission simple, you may limit the management cost explosion. In a sense it is sad because it means you are so cheap, no one cares if you fail (other than you). As soon as the financial sources start to care, you get into NASA/ESA budget regions. So maybe it is the best way to proceed, making multiple smaller mission.
The Canadian ACE/SCISAT mission also achieved something similar. Its a very simple science satellite, with only two instruments. The costs were ridiculous and the time from planing to launch was extreme short. Considering it flew totally new and revolutionary instrument designs, I find that quite amazing. The mission as now significantly outlived its initial planning and is one of the most successful scientific earth observing mission. So much for those who think it has something to do with the costs of engineering in India. I doubt the Canadians engineers are much cheaper than the American ones. The key to success was to keep and small.