I have came accross a great video about monetising your blog.
Everyone interested should take a look to this.

In short words:
- AdSense is not the best choice to monetise the blogs, you don’t have the control on what is displayed on your site, even if you can block some websites (if you see it).
- Affiliate links is far more better
- Direct Ad Sales - more common, you see the ad, the content, more simple. It’s very good for niche market, it’s the best to monetise the blogs

You can start to monetise the blog whenever you want, you can even start to seel from as low as 1 USD, you have to start somewhere.
Based on the market you can determine the price.