"Reasonable" is generally relative to the size and resources of the company. It certainly doesn't mean "it has to be free".
If it's a big company with hundreds or thousands of employees, something like having a cubicle with taller walls installed would probably be "reasonable." Moving you into a corner, and/or waiving certain rules like "no headphones" that don't cost anything, and don't interfere with performing your job, I would think are always "reasonable."
One suggestion, though: Talk to HR first, and work out what's reasonable with them. That way you can discuss the issues without your boss trying to steer things towards what's cheap/convenient for them. I've already had a talk with our department's HR rep, and she basically said "come talk to me." We'd work out what needed to be done, and they'd handle the communication with my manager. Basically it comes through as "HR approved accommodation - do it and don't ask questions that will get us sued." (In general, always make sure HR knows that you have a disability, and what it is. This is really important if you get disciplined or fired for it - it will be too late then!)
This all assumes you're not working for a very small company, where the laws are different.
Also, if they DO just say, "screw it, you're fired!" talk to a lawyer immediately - you probably have a nice payday coming.