Beam uses fiber optic network which is pretty expensive... I am not it is all that easy to generate fund for such an infrastructure all over India...
Sorry, but that is so not true.
While copper is cheaper and easier to repair (if you do a hodge-podge job of a repair, that is - I'm talking about twisting the wires together then putting some electrical tape around them), fiber is by far cheaper overall, with the added benefit that you don't need as many signal repeaters - one every 60-80km is enough compared to one every 5-ish km for copper. You also need far less actual fiber cable to serve the same amount of customers compared to copper too, and the speeds are exponentially faster.
So, the cost of building is roughly the same, but the operational expenses are way down by comparison. The only things more expensive than copper are the actual tools used for splicing as well as the ends - a few dollars for a fiber-optic plug compared to a few cents for an RJ-45 plug.
Interestingly, the largest cost for our build out comes from not fiber, not equipment, not tools... but right of way/government charges and in some cases, digging/civil works. I expect the same is true for the folks at Beam.