A line marked-off in fathoms with a weight on the end?
Seriously: You're not going to get much less expensive than $270 for a depth sounder.
FWIW: Our boat came with Raymarine ST60 (or maybe 60+) depth sounder and knot meter. They seem to work well.
If you go for "last years" model of GPS: Yes, you can kind of get it "all in one," more cheaply: GPS, sonar (which will give you depth) and kind-of-a-knotmeter (SOG: Speed Over Ground). Our Garmin GPSMap 498C with external antenna and in-hull transducer was about $620, shipped, which is only a bit more than the MSRP for just one
of our two Raymarine instruments. Plus it came with built-in cartography. Still: I wouldn't want to be w/o my separate instruments.