Go with boxes or battlechests. $20 WoW, $20 expansion(If anyone charges more fuck them; I know gamestop is still charging like 30-40 for BC while walmart sells both at $20, and some places have battlechests on sale for $30)

Main account clicks "Refer-a-Friend" link at Acct Management page, sends email to alt acct
Alt acct signs up for 10-day trial through the refer-a-friend link/code
Alt acct registers WoW box's CDKey = 1st month free after 10day trial is over
Alt acct passes 10 days + 1st month and pays for first PAID month = Main acct gets 1 month free via the Recruit-a-Friend setup.

Repeat for each account to ease the monetary burden ;) Boxless trials upgraded don't come with the free month that I know of so it's $15-20 wellspent and what you'd spend anyways on the monthly regardless, you just have physical CDkeys to prove your ownership with.