Well... this answer is going to assume a lot... but:
If you already have all the plumbing in place, and the wiring is existing and in good shape.....
Your looking at the cost of the pump and filter (plus markup) and the cost of labor and whatever materials
are necessary.
If you have an existing pump, there shouldn't be a great need to replace all that much of the plumbing
materials. Maybe a new PVC elbow or two and some misc stuff.. but nothing major.
Also depends on which pump / filter combo you go with. So lets leave that out. You can get systems ranging
from $500 or so and go way on up from there. Assume they will mark up the pump unless your providing it.
Labor, well that is going to be kinda hard to say. Depends on your area, and if your getting a local handyman
to do it or a professional pool builder / servicer.
So... all in all... it depends.
Assume likely 15 to 20% markup on the pump / filter hardware if they supply it
Assume something like $300 to $500 for the labor, but it could vary depending on exactly who you have do the work.
I'd be figuring on spending at least $1,000 (and that is considering a somewhat cheap pump). If your going to go with
a variable speed pentair like you had... probably closer to $2,000 by the time it's all done.