You can draw a fairly straight line between the points, except for the Family Tree DNA individual tests and the extraordinarily expensive 10 marker test offered by Oxford Ancestors. Looking at points falling below the line makes it clear that Relative Genetics, shown with an open circle, offers the lowest cost test, 18 markers for $US 95.
Cost and number of markers are not the only factors. Do you want the company to keep your DNA so you can order additional tests as the science advances and your, or your descendants, needs develop?
What reputation does the company have in being responsive to the client? Few folks get much insight from a tabulation of their analytical results. Most genealogists can benefit from a bit of advice to facilitate understanding the data. I only have experience with FTDNA and have found they routinely answer queries promptly, although not always accurately when it comes to responses on the timeframe to deliver delayed results.
Finally, although there are now several free public databases you can use to compare your results with others and so find genetic cousins, the FTDNA proprietary database which provides automatic matching for their clients is the largest, meaning a greater chance of finding a match. That's a major advantage and, along with reputation and ease of access, likely accounts for the company being able to make the claim that "nine out of ten genealogists choose FTDNA."