I suggest you look on eBay.
The price for the A1200 itself would be less than for one which has a RAM upgrade, an accelerator or other upgrades, such as a CF card hard drive, and/or a Gotek floppy drive emulator.
The disks aren't worth much, because all classic software is available for download as ADF files, but the disks may be unreadable and your floppy drive may no longer work.
You also have to take into account the price of the monitor separately. I think this monitor is a badged version of a Philips 8833 monitor. After that, you add both prices together.
Saying it's for collection only reduces the price by reducing the number of potential buyers. Sending a monitor by post or some other delivery service is very risky. The monitor stands a good chance of no longer working when it arrives at the other end.
Unfortunately, I can't give you a specific price.