Until you can write down on paper step by step and you clearly understand the model of the publishers system you should not spend any money.
Here is how you can learn and clearly understand publishers models and systems behind how they make money.
Go to different websites and do the following:
Click on ads you see around the internet and save the copies (headline, image, description, sub headlines) as pictures or on a piece of paper
Then write down these things in answer form on a piece of paper.
1) What ad units and where do they come from.
2) How much do those sources pay on average per CPM/CPC
3) Where is the traffic coming from and how?
4) What things did the publisher your studying do different than a similar publisher you've studied.
5) Ask yourself, can I duplicate what they are doing using a different niche.
You can use websites like similarweb.com to see where traffic comes from and where it goes when it leaves. I advise you to go to every one of those URLs and learn about what those companies are and how they are being used at the website/publisher your researching.
Study what other people are doing in a scientific manner. Ask yourself questions and find rational solid answers and write things down, you can throw them away after your done, but write them down.
This is a very technical industry. You have to look at it in orderly steps. Bring your throughts back to pen and paper and follow these tips.