Killer Juice has a sale every Weds-Sun (usually starting Weds afternoon) of 15ml bottles for either .99 or 1.99.
FastTech and MountBaker are pretty inexpensive places to get juice.
But some e-liquids get up near $1/ml. Many people think the more expensive juices are worth it, many don't. Find what works for you, start out with small quantities and small bottles from any given vendor, and make sure your head and PV are working well before you go on the hunt for juice!