For a 24" monitor, there are 6 CCFL tubes - and the cost is going to be $75 - $125 for the kit...although you might get lucky and find it cheaper. As a rule of thumb, if a monitor is more than 3 years old OR the repair cost is more than 50% of the cost of a new one, I purchase a new monitor. Unfortunately, they are made so cheaply now that they have become "plug and throw away".
A new monitor will be about $150-$200, maybe less....