You will absolutely need the lesson plans, and the worksheets, and an abacus for each child (although I did make our own to start with.)
You will need Place Value Cards for each child (you can print some off that you find on the internet - but know that the RS ones are much nicer, durable, and easier to use.
The Geared clock is nice (recommended one per child) - you could make due with a paper clock with movable arms - it just doesn't show how the hour hand moves a bit with the minute hand...)
Base 10 picture cards (the site doesn't recommend one per child - but if doing the lesson together, I do...) - can probably find some on the internet to use - the RS ones are really nice again...
Abacus tiles - could honestly make due with the base 10 picture card for 100...
Cards for Math Games - REQUIRED
Geoboards - I did find some virtual ones online - it is only used in level B for a few lessons - but my kids love them
Wooden Cubes - we are almost done level B - and I think they were used in 1 or 2 lessons? could use any regular-sized cubes in this level....
Colored Tiles - We used these a lot, although you might be able to make due... note that these are 1" on a side, and do get used to measure stuff at some point. Used for counting, patterns, etc.
Math Balance - My kids love this - there is a virtual one online somewhere that you could use, but they might not "get" it so much. That said, it is really only used for a few lessons, unless your child chooses to use it more.
Tally Sticks (craft sticks / popsicle sticks - get at the store)
Geometry Reflector - used a few times - a pocket mirror would probably work
Yellow is the Sun CD - songs are also on the RS website
Game Instructional DVD - shows how to play some of the games - note that I think they are all also on Youtube.
SO - This might help you do things a little cheaper - not sure how much you save with that. You can call RS to see if they have any "looked at" stock (looked at at trade show, or returned without being used) that you can buy at a discount.