People are naturally drawn to the low price alternative when trying to solve a problem. When you are talking about PD flow meters, that choice would be a gear style meter. Gear meters might be the answer that you seek, but let's compare them to a piston meter first - before we fall in love with their lower price.
For starters, the gaps on the sides of the gears are about 5 times larger than the gap between the piston and the meter body. If you are measuring a lower viscosity fluid, like diesel fuel, gasoline or some other light hydrocarbons, these gaps look like an expressway for the thin fluid to slip through the meter. This internal leakage will underreport the true flow rate and may lead to the gears stalling in their rotation at the lower flow rates. To disguise this effect, most companies express the accuracy and range of a gear meter when operating on a 30 cps fluid. While not dishonest, it can trick customers into using gear meters well below the best working range.