As far as I know nowadays the safest  and the cheapest way of correct linkbuilding is crowd marketing. From the very beginning, I had some hesitations about such a method, but now I am using crowd links along with PBN links and always get the positive result in several niches. There are few companies that can provide you with high-quality crowd backlinks, you can try some of them and just compare the result in the end. I have chosen linkology.biz, for example, because they have the most affordable prices and really nice quality of those crowd links and of donors where they are being placed. I haven't used any other service for the last half a year, that's why I can't recommend them to you.