A poor one, head only, unserviced (but complete) around £1000-1200.
A very good one, with correct bracelet, fully serviced, say £2500-2800.
These are private sales. Dealers are more.
Condition is king, but bezels are extremely hard to find so do not buy a cheap one without a bezel hoping you’ll find one for a couple of hundred quid - you won’t.
Also, the correct bracelets are £350-550 on their own.
Be aware that the majority of seamaster 120’s for sale are fake. Finding a good one for around £2500 takes patience. If you want one quickly you’ll have to pay more.