Minimizing your DNA testing cost is not simply a matter of finding the lowest price. Testing companies vary considerably in terms of value offered.
I believe that Family Tree DNA is the best one-stop choice in genetic genealogy, including Y-DNA, mtDNA, and autosomal DNA testing for genealogy purposes. To learn why see my FTDNA Review page.
Two other good tests (both autosomal) are 23andMe and AncestryDNA.
When I learn about sales from the major DNA testing companies, I announce them in my blog. Go to the What's New page on this site to check for sale pricing and other news. If you subscribe to my blog, you will know when I report on such sales.
On rare occasions, Family Tree DNA will quietly offer discounts through their DNA Surname projects. You will not see the promotional prices if you simply go to the FTDNA web site and click on Products. They design these offers to help the administrators of DNA surname projects get more members. So you have to join one of the projects to get the promotional price.
DNA surname projects are a great way to combine DNA testing with genealogy. I am the administrator of a DNA Surname Project and I appreciate all that Family Tree DNA does to help these groups.