Bandwidth (measured in MHz) is the biggest factor that affects the cost. There is no point wasting money on bandwidth that you are not going to use.
For just about all hobbyist uses, 10 MHz is fine. Most communications systems, such as CAN and LIN on vehicles, as well as I2C and SPI between ICs can be read without any trouble at all with a 10 MHz oscilloscope.
2 channels is usually enough as it lets you see the timing difference between two signals. If you have more than two signals, even with a two channel oscilloscope you can make several readings to work out the timings.
Like the frequency question, it depends on what you are doing. If you really need to see all the ignition / injection pulses on a V8 engine at the same time, you need 8 channels, and deep pockets.
For the 10 MHz bandwidth, Picoscope offer good value for money.