I got mine on Ebay - for £175.00 plus carriage.
It is the PD1 which has the display for the heartrate. (I can't do links but the seller was 'Dopplermike' And I received it amazingly fast) I recommend it completely, we used it from 10 weeks, (I'm 14 now) and it is a godsend. You can sell it on ebay 2nd hand, I have seen them go for £150=
I can find both heartbeats relly fast now (twins), within 30 seconds or so.
I don't rate the cheaper £40 one either, could never find ds's heartrate, not even at 30 odd weeks.