“Better” is a relative term. I doubt you’ll find a cheaper z-axis than the stock kit. The stock z-axis works, although it requires careful adjustment of the router base (loose enough that the motor body slides smoothly, tight enough that it doesn’t rattle), and a bungee cord to hold the motor body down. The plastic nut that rides on the lead screw has a bit of slop.
A z-axis based on a lead screw or ball screw with a better quality nut will probably give more accurate z movement. It might even be a bit better in x and y because the router base wasn’t really designed for how we use it. Someone else will have to comment on price, but I’m sure more than stock. The Meticulous Z-Axis is a good example, although others have their own designs as well.