I am new to this forum. I hope my post is still useful.
Context would be helpful. "At the low end, configuring the router will probably cost staff time" sounds like something that would be said during a business meeting.
"At the low end", or "On the low end". 'The low end' is making referance to a projection that is attempting to estimate (or accurately portrays) the costs of configuring the router. The projection would have a range of costs, from the lowest cost (the low end) to the highest cost (the high end).
The sentence could be changed to:
"Probably the configuration of the router will at least cost staff time."
I think that the sentence in my example is more clear to the meaning of the sentence you provided, however the sentence you provided has an additional meaning that mine does not have. By saying 'at the low end' you imply that there is a 'high end' and you know what it is. My example sentence is more uncertain about the cost of configuring the routers beyond the probable lowest cost.
Additionally, my example would be used in a different context than the sentence you are having trouble with. 'Probably the configuration of the router will at least cost staff time' is an answer to the question "How much will the configuration of the router cost?". "At the low end, configuring the router will probably cost staff time" sounds like it would be used in an arguement or an attempt to convince someone to accept the cost of the project.
Either way, the sentence leaves out how much configuring the routers could be at the high end.