There are lots of WAHMs that make great diapers that are cheaper. The diapers you are talking about (Bum Genius, Happy Heiny's, etc.) are great and worth the investment, but if you're not ready for that, then don't do it.
A much more economical choice is to use prefolds and covers. Depending on where you go online, you can get good Chinese or Indian prefolds (stay away from the Gerber ones in walmart. they're not worth your time!) for $18-24 plus shipping, and you can get away with 3 or 4 PUL covers (at around $10-11 each). The 20 month old will probably need "regular" or "premium" sized diapers and you could do 12, and the babiest baby needs "infant". These will last until 15 lbs (or more if you just trifold in a cover). The littlest will need about 18, and you'll probably be washing every day or day and a half.
Prefolds are the cheapest way to go, but they are not "stay dry" like pockets and AIOs. Perhaps you can start off with a couple of the bigger name ones and mostly prefolds, and slowly add to your stash.
Keep looking on craigslist. I recently sold 24 infant Chinese prefolds (diaper service quality) for $40, no shipping. They are there, you just have to keep looking!
Also try www.clothdiapernation.com and www.diaperswappers.com They are cloth diapering and natural parenting forums where people discuss CDing, and there is a For Sale Or Trade (FSOT) section. You'll have to register, but it's a good way to get deals. Cloth diapers can be sanitized, so used is an ok solution.
Please send me a message if you have any questions. I have cloth diapered 3 children. One is out of diapers (except at night), my 21 month old is still fully in diapers, and my baby (3 months) is also. I've used all kinds of diapers. My daughter wasn't in cloth until 4 months, but the new baby has been in cloth since Day 1. I have lots of experience and would happily answer questions.