Cost of linear actuators

I have an application where I have to move a 3 lb object in a 3 ft cubic space. I am not looking at high performance or high accuracy.
It can take as much as 30 seconds to traverse the 3 ft distance and the accuracy could be no better than 0.020 or 0.050 inch (possibly the spec could be even looser).
Linear actuators would appear to be the best solution, but I am finding that they are hundreds or even thousands of dollars. The product I want to build will have to cost no more than $2,000 for a 3 axis device with controller, overhead and profit.
What is it in a linear actuator that costs so much that you could not build a low cost version using a threaded rod, servo or stepper motor and an extruded frame for a lot less?
Does anyone know of a linear actuator that can provide the kind of performance I am looking for for a couple hundred dollars?
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