Why D90 is high priced

d90 is for professional use ie it is better in term of color accuracy, faster start up and focusing, almost no shutter lag, internal focus motor for wider range of lenses, taking more pictures per second, etc-etc. the d5100 is for beginer. it provide more option for people who wanted to explore photography but with limitation like slower, no focus motor, etc (see comparison). the higher megapixil is due to the cost for production has gone down means cheaper nowadays. the d90 is an older model. hope this help.