My son is interested in getting a monitor of some sort. I know nothing except that they get very big..
Not all of them get big sizes range from very small to very big.
How much on average does it cost to feed a full grown monitor and at what age is full grown...
Cost of food depends of course on which monitor your feeding. Something like an ackie will eat mostly insects, some pinkies.
Something like a water monitor will eat rats, rabbits, chickens.
Everything inbetween that will eat mice, rats, chicks, insects.
I toss my ackies a handful of crickets daily, my waters eat 2 large rats, or a chicken every other day or so. There is no set schedule so sometimes they eat more sometimes less.
I need to find out which ones are friendliest as well as which ones get what size..
Tempermants vary within individuals as well as species. If you choose a species thats known to be docile it will help. But be prepared as they all can or may bite, be wiggly, or just simply not want you holding and carrying it around.
As far as which gets what size, it would take awhile to type all of them out and what size they range. That information is probly already up someplace so just do some reasearch.
Otherwise try to pin point more of what your looking for in a monitor then we may be able to help. Leaving your question as it is worded is a very broad spectrum. Try narrowing it down a bit by what you expect out of a monitor. What colors, size, cage, diet, temperment, etc. that you are looking for.