I agree with mlward3. I bought mine from ebay for just under sixty bucks including shipping. It is a sonoline b also. I got it when I was 10w 3d and could hear the heartbeat then! It has a couple of different modes for telling what the heartrate is. Super easy to use and read!! Totally recommend it! Good luck!
I went on eBay bought sonoline b for about 70$ I love it! Found heartbeat right at 9 wks!
I used bellybeats last time and it worked for me around 10 weeks. This time I bought one off ebay the sonoline it's the same as the sonotrax it's just like the one from bellybeats. It depends on how long you plan on using. We paid $45 a month for the rental with a digital display for 3 months. This time I bought one for $140, but we can also use it for our next pregnancy or even turn around and sell it back on ebay.
I rented one from bellybeats.com and it works awesome i was able to use it at 9wks its just like the ones used at the doctors....hope this helped good luck :)