howdy Blackrose and welcome to the forums!

costs? that depends on your hardware and your expectations. if you have a dual-core and want to 3 box (or less), you should not need any new hardware. quad cores are good for 5-boxing, but you can 5-box on a dual-core... but with lower fps. your graphics card, OS, and system memory will impact here... more info is required.

if you already have one wow account now, you'll need to get a new one for each character you want to run. get 1x wow battlechest for each new account. (and extra for wotlk once you bump against 70)

only piece of the puzzle left is your boxing software. some are free, others have a one-time fee.

your costs:
$25-40 * (number of wows) wowcraft battle chest
$15 * (number of wows) monthly fee
$0-19.99 multiboxing software (**cough** keyclone)

they usually have memorial day sales... watch the papers.

as for making $$ to pay for your new habit... being 15 shouldn't get in the way. i use to mow lawns on the weekends during the summer... good for $20/lawn these days i'm sure. one lawn a week would cover it... but i'd get as many lawn gigs as you can find, to get some new hardware. there is also the summertime job... subway or whatever local businesses you have (have you had a job yet? if not, i can give you tips to get one)... stock up on game cards during the summer and your wows will be paid for a year. you'd only need 6 per wow to cover a year.

if you have any questions, please feel free to drop me a PM anytime.

good luck and welcome to the addiction!

Rob (keyclone developer)