What he said ^^^^^^
The reason Monoblock / 1-channel amps are the most expensive is that they are designed to power subwoofers which are either require serious power, or require top quality sound input to run...
2-channel are the cheapest as they arethe easiest/most produced, and run just about everything from a set of 6x9s, or even run a small sub if they are "crossed", which means using both channels for 1 speaker/sub...
4-channel are that bit more expensive as they do twice as much as a 2-channel, with most giving the option to run just the 2 speakers, or again, cross the 4 and run 1 speaker/sub..
HTH a bit...