Less than this Volvo owes me I would say
All depends on provenance and/or how certain you are that the block is good enough. Dan S for example has been through what 3 blocks(?) trying to find a suitable one for his tastes.
I would pay (and have paid) £500-£750 for a complete engine with heads with a shagged block (depends on how complete). A running complete engine with a good chance of a having a good block would be in the region of £1250-£2500 I would say, with the £1250 end probably pushing the definition of "running" and "complete".
Last sale that I know of a "good" engine" was a complete (previously running) engine (with DeLorean crankcase and sump - and full history) for £2000(+VAT), and that was 2 years ago. Even that engine was then completely stripped down and rebuilt (and loads of stuff replaced on it!) and is now powering a car successfully.
These correct (i.e. not the B280) old Volvos are becoming really rare now. With the "cool factor" of Luther etc
there will come a time where people are buying old DeLoreans to rip the engine out of to stick in Volvos I reckon