Marc,
The cheapest way to use 'in-ear-monitoring' :
Buy a couple of hi-Q earphones (discman alike), insert them into the phones-out of the mixingconsole.... c'est tout !!
(Total costs about 100 USD ?

The pro-systems however are wireless so they need a transmitter, a license, a battery-powered receiver for all musicians and aerplugs..... pretty expensive (2000 USD per unit ?)

Roel