Their sales data is not available. The MinION program does not seem to be designed to finance the company, though it might actually be profitable. The PromethION on the other hand, is selling faster than they can ship the devices.
Their valuation is not about short term sales. Their technology really is somewhat of a holy grail. There is no "simpler" way to read DNA in terms of technology. Everything else involves complicated biochemistry. Illumina beats Nanopore Technologies right now because of the scale of their machines. But they can't scale them down much further without losing performance. The most efficient Illumina machines will always be cupboards or bigger.
No competitor can come up with something better than ONT. Nothing smaller, for certain. The only way to beat Oxford Nanopore in the future will be to implement a very similar nanopore sequencing scheme.