what's the ratio between the number of reviews and purchases of your app?

Mine is about 3.62%(67/1849) for one of my not free apps .I think the ratiois a little bit of low.

Only one in 27 publish a review.

Is that normal to you guys?

I wouldn't consider that being anywhere low really. Generally people are not that interested on writing reviews, especially if all old ones are in English and the person downloading the stuff is not English native speaker.

I think most users is too lazy to like review because this how i also feel when using or downloading the apps :D .One of my free app got 70000 download but only 120 review

I think this a good percentage. For my apps the value is lower (~1 to 3%). A big percentage are questions or bug reports which might be better suited to be communicated by other means than a review.