Please explain your doubt more, i cant understand what you want to ask.

Nath, with AdSense you need to choose a site theme that is going to attract ads that a reader will click. You then need to have a think about how much an advertiser would value a reader from your blog.

A lead must be worth enough for an advertiser to make a return on. A $100 coffee machine for example won't get a great for the advertiser (working on a 20% profit margin) so they won't pay much per click. If the advertiser was selling $1000 coffee machines then suddenly you attract advertisers with more room for advertising spend.

You also need to avoid niches where advertisers will be limited. I'm not familiar with the asthma market but it would need to have doctors and clinics advertising to win your business. It may have a limited amount of doctors will to advertise so you could risk getting irrelevant ads - and irrelevant ads do not get clicked.

Brainstorm some markets and products which an advertiser will pay a lot for the lead. You can then go to Google and find other blogs and sites writing about those subjects AND using Adsense, and when you visit those sites ask yourself: would you click on one of those ads if you were looking for information on 'chosen subject'?

Fraggler has provided an excellent answer - there's just 1 thing that I would add:

You can get good Adsense earnings from $100 products if the product lends itself to a lot of repeat business.

For example, taking the $100 Coffee machine above - if it came with special filters that can only be purchased from the same supplier, then the total lifetime value of the sale is worth much more than $100 to the advertiser.

A better example is software that provides in-app purchases for plug-ins and add-ons etc. In many cases the customer will spend significantly more on the additional purchases than they did to get the basic software.

So if you're in an industry where they sell the razor cheap - but charge through the nose for replacement blades, then you can still do quite well even though the headline price of the item is low.

Actually working with any types of keyword will be suitable for you I think. You are confused about which one you should use. Look, you can work with high competitive keywords as well as low competitive keywords. The choice is yours. But, here I can prefer you to check out the following rules for that -

4 Unusual Ways To Research Low Competitive Keywords Quickly

What is more you should be more specific about your question to get the exact answer always. Will try to share you something always.