I am preparing a grant application, for a project in physics (experimental optics).
So, my question is: how much would it be reasonable to plan for "publication expenses" in the budget, in one year?
I plan to publish one article in one year.
Well the easy answer is to look at the possible journals you would publish in and see what they typical charge.
But you have to be careful, I've found some funding agencies wont fund publication charges (either directly, or because that funding comes from a separate agreement with a University). The funder may also have a cap for how much can be charged to each line item (like publication charges).
As with all funding questions talk to your University, as they have people that specialize in this and know how to deal with each funding body (as every funder seems to want to do things slightly differently). Also talk to other people in your department (they might let you see their past grant applications) to get an idea of what you can and can not charge.