I wonder if there is anyone here using the third method - paying money to obtain good backlinks? Can you share your experiences?
Should I invest on it?
I guess this is a delicate topic - many people never would admit that they are buying links, but actually a lot of people do so.
Paid links could be valuable if you achieve high quality links that will boost your rankings and have impact on your earnings. There are quite a lot providers offering some kind of backlink services.
1. marketplaces to trade backlinks: They are made for backlink seller and buyer
2. Backlink seller
3. Linkbuilding agencies
Beware of offers like backlink packages (200 backlinks for 39$) or automatic software solutions. They aren´t worth the money you pay for it (no link value) and it´s easy for google to detect them.
Go for quality links, from relevant sites with good and unique content, from authority sites with trust, vary your backlink anchor texts, etc. If you can´t get them for free or by link baits, try to get them at teliad.com - you can register for free and there are some tools that help you to search for appropriate links.