We have the Hi BeBe doppler. It's 3Mhz. I believe it was roughly $100 (on sale at the time) and we used it pretty frequently. I liked it a lot but once I started feeling movement, I didn't use it again. I can't even tell you where it is right now... in a box somewhere, probably.
IMO, if you're in your 2nd tri, it's kind of pointless.