I've seen ones with smashed dials make £100GBP and mint ones make £600GBP + and I've also seen a chap get £550 for one made up out of aftermarket parts (located in Bath UK so you've been warned! Don't bee jooped!). Be very fussy when buying Seiko chrono's as a lot are made up out of aftermarket parts and others banged together using old junk watches.
As Mike says, £100-£1000. It really depends on condition, both cosmetically and internally. It's a good idea to buy these from a seller that has checked it over, serviced it and made sure all the gaskets are present and correct.
I buy (and sell) these watches and you would not believe how many I get in with missing parts, especially missing pusher gaskets.
The Tokei Zara doesn't seem to command as high a price as it should in IMO, it's perhaps the best 6138 'bargain' out there at the moment....so damn nice.but do expect to pay for a really good one