Hi there chongjasmine!
What exactly is it that you're promoting that is 'writing' niche related?
I did some research for the keyword 'writing', and that keyword alone would get you loads of traffic! As would a few of the Ad Group Ideas. Very intriguing niche! Maybe all of the marketers have skipped past the keyword 'writing' because they're scared to compete? I hope not, because just the keyword 'writing' could easily get you 200 potential customers per day, and only cost you $0.20-$0.75 for each lead! Meaning, it all comes down to presentation, product, closing, and/or giveaway for a list build. What's your product or service?
The results for a keyword such as 'writing' are gigantic (more than one billion), and not only that, but the keyword isn't vague in any way at all. You're either interested in writing, or you're a reader. And the only other set of people that could use 'writing' as a keyword would be somebody looking for a person to do some writing.. are Most people that search for 'writing' are probably Writers, or interested in becoming one. Right?
So when you said, "However, looking at google adsense, the cost is high per click.", are you referring to Adsense because you're getting paid by Adsense, and the clicks for the writing niche ads have been paying you good and that's how you're defining it as a high per click niche?
Now depending on the type of writing you're into advertising for, if you make an ad that presents a price, and you use negative keywords in keyword planner (free, no charge, no cost, freebie, freeware, cheap, etc..), then I seriously see opportunity for you to at-least build a list from the keyword 'writing' all by itself. Be sure to give away something that is irresistible to your visitors when you build your list. That way you can follow up on the original offer that they, originally, arrived to your page for.
You should also consider finding a keyword, or two, that you can search engine optimize a page, or two, around to get some organic traffic. You will be able to get your word/product out while it happens. This method will, obviously, take a little time, but it's worth the long-term investment of it, so be sure to SEO you a few pages using keywords that can put you up on search results fast! Keywords that have high searches, but low amount of search engine results.
So what is the product that you're promoting?
