Rubber buttons missing may indicate that the battery has gone flat at some point. It's usual to remove the buttons to expose the normal lock mechanism that can be manually opened by the 'flip out' key section of the remote fob.
As for prices, ring Dixon Nissan - no point in even guessing prices.
If you decide to go further with this car (or any other Micra with keyless entry), try taking it for a run of a few miles or so with the doors locked from the inside. It's not uncommon for the boot to unlock itself while driving (it'll bring up a warning light on the dash for 'door open').
It's - unfortunately - not unusual to have to have the tailgate switch replaced, so make sure you've got some warranty, or a good price on the car. One of the consequences of Renault taking part ownership of Nissan I'm afraid......