Dig a hole about 2' deep widened at the bottom, through the center of the hole drive in 4' long screw together rods until refusal-then cut off rod just above ground level, fill the hole up with concrete to 8" of the top of the rod, set a 6 or 8" diameter sonnet tube 10" long 2" into the concrete and fill up the last 8" with concrete, set the cap on the rod and finish the concrete to drain. Tamp dirt around the sonnet tube. Be sure to have something magnetic in the concrete.
Usually takes 1-1/2 bags.
The cost of a premade DOT concrete monument was about $150 last time I checked. Then you need to get them delivered.
It just seems easier to do it on site. If you build a monument off site you still have to dig the hole so might as well use the ground as the form, saves a few steps.
One option isn't exactly a concrete monument, but a cap on a 2-1/2" pipe can be turned over and filled with concrete-it's pretty close and a lot easier to deal with.