A ballpark figure
a rough estimate
e.g. I need some work doing on my house but I have no idea how much it would cost and I have little savings so I asked a builder for a ballpark figure.
The idiom comes from sports commentary (probably baseball) in the US where a commentator would try to work out the size of the crowd.
The origin of this phrase comes from how a commentator would give an estimate of the number of spectators by just looking around. Speculated to have started in America through baseball but it is now a popular way of speaking throughout the world.