shimano/bosch mid drives seem to start at $2600, but they have the most natural pedal assist(through torque sensors in the crank), good torque, and are geared through the chain, so they'll take advantage of all the speeds on a cassette. As far as I know, these are the best option if you want to have the most "natural" pedal assist experience possible, because the torque sensor allows the bike to exactly multiply your power, rather than make guesses using cadence sensors.
Chinese DIY mid drives are powerful, but they typically lack torque sensing, so they are less enjoyable to pedal as a bike. The tsdz2 kit is said to have torque sensing, and thus more natural pedal assist, but it's hard to find direct comparisons against bosch or shimano ebikes. These are about $800 to $1000 to retrofit an existing bike, yourself.
Hub motor bikes are much cheaper, but are much less efficient at low speeds, meaning that they're not typically good choices for hills. There are *geared* hub motors, which add a planetary gearbox in the hub, which eliminate this problem. These are essentially the only pre-built ebikes you'll find below $2000.
