Detecting and counting metal plates
Optical break the beam type may be best though it depends on the contamination considerations. They must be kept clean at all times.

As you say, industrial inductive, capacitive and hall effect sensors are not cheap. However, they are extremely rugged and simple to use.

You do not describe what "Coming by my sensor" means. If the plates are on rollers then something as simple as a lever-arm micro-switch under the plate path may be sufficient.

However, without a lot more information it is almost impossible to answer this question. Even then, a lot if it is preference.

Question Edit  - Answer Update

Ok, so that is an entirely different scenario. 

In that case I would use optical break the beam type sensing.



In order to make such a system pretty much insensitive to ambient light, it is best to modulate the IR LED at some frequency and use the receiver photo-led in a non-saturated analog mode and detect the frequency modulation arriving at the output of that.

If you want to make it really sexy you can use a fibre-optic transceiver on your board and just run up some dual optical fibre up to the sensing points. That method also gives you much better EMI immunity.

At those distances though you probably need to enclose the sensing side, and have a plate with a slot or small hole in it at some distance in front of it such that only light coming from the general direction of the LED will fall on the sensor.

An alternative is a hall effect sensor, but they have to get pretty close to use them, and arranging the magnetics to make that work can be difficult. Other issue will the hall sensor in the sides of the plates need to be physically similar and stacked perfectly.