I have a angel sounds jumper doppler, and I tell you what, it's fab!! I paid £20 including delivery on Amazon 2 years back. It arrived next day too.
It doesn't have a display but picked up both my pregnancies at 8 weeks 1 day with first, and 8 weeks to the day with the second (7 weeks 3 days according to scan dates?! But not going by that!!)
My midwives wouldn't even pick up the baby's heartbeat at 11 weeks at my booking in app...
xx