It is a good question. First of all, never buy any backlinks from webmasters that own spammy sites, because it will do more harm than good. So always ask for the list of sites that you will get a backlink on.
I can tell you that backlinks on very quality sites are pretty much always expensive unless it is a quality site in a small niche.
I would suggest searching for sites in your niche and contact webmasters from the best ranking websites.
But remember that it is better to do not have any backlinks than to have backlinks from dodgy sites.