The most economical way for rural roofing would be to gather locally available materials (preferably for free i.e., they can be just adapted and used rather than bought) and build a sound and durable roof out it. Durability is much important than just cheapness because if it’s not durable, it’ll need oft replacement, which will make it uneconomical.
Depending on the location and the time of the year, the locally available durable materials could be bamboo, mud, hemp, straw, clay, terracotta, etc. These could be used in combination as well. Like mud + bamboo roofing is quite common. It’s surprisingly strong and also insulates well.
Other non-natural yet locally available cheap materials can be ton sheets, waste plastic sheets, asbestos sheets, or unused wood.
Remember, the idea is more focussed on cutting down the cost of procurement and transportation by getting materials for free locally rather than looking for the cheapest material overlooking durability.
Also, it’s best if labor is not employed, and roofing is done by oneself. These kinds of jobs are rurally common nonetheless.