In fact, you can invest Bitcoin quantitatively every day. You only need to invest once to obtain long-term returns. As long as your machine is up and running, it generates a certain amount of bitcoins every day, and the benefits are endless. Acquiring bitcoins this way does not require you to watch the price every day, wait for the price to rise later, and directly trade in cash. Relatively speaking, cloud mining is the lowest cost.
I thought you are referring about actual bitcoin mining where a person buy a rig/asic, set it up connect to the mining pool and mine bitcoin, but then it turns out you are promoting cloud mining. I do not think it is the lowest cost because you need to top up or buy extra hashes as time goes by to compete with the shares on the pool. Aside from that, buying hashes on cloud mining services had been proven not profitable and have the highest risk of being a scam.
I would go for buying Bitcoin than mining because it is the most efficient way to acquire Bitcoin. No maintenance, no noise, no huge electric bill.