So the most advanced one is also the cheapest?

Well not exactly technically the R3 astromech was semi more advanced than the R2 it was less reliable and would constantly break down and it wasn't even made for starfighters to help them get to hyperspace it was more usually made for landspeeders and small land vehicles though it could be modified for starships

Not sure you're getting that from, it's in neither the Canon nor Legends version of its article.