It's difficult to say. It depends on individual factors. However, I would say that maybe 50 percent of profits should go back into marketing. However, I'm sure some companies would put more into it. I don't know if some companies would put less.
This is difficult to say but the thing is, since marketing is a continuous process, a portion of the profits should always be set aside. If you check elsewhere, you will read that 7 or 8 percent of the gross revenue should be spent on marketing and advertising.
I'd spend atleast 100$ a month for the PPC Ads. If you have landing page and the ad setup, running the same ads won't take much time. Also you can reach more people with PPC ads. You may have to work harder on some of the other branding options like social media. And you can double effort there without having to spend much money. But it all works out in the end.
We are definitely looking at it as an investment but you should also note that if marketing isn't done in the right way, then the desired results will not be achieved. You actually have to apportion a percentage of the revenue towards this cause.
You need to find correct ways to aim for your campaigns. Sites like fiverr can offer you some basic ideas, but keep in mind they use a lot of bots, so only check out professional sellers and what they offer. Most of them usually offer multiple deals. You can test the sellers quality by getting the cheapest package and if it proves to be good you can try the more pricey ones as they usually offer more services as well