As far as the cost factor, I purchased my lesson books used. I know I’ll get most of my money back when it is time to sell them. The best deals I got were from the RS yahoo group that suzukimom mentioned. I bought the teacher’s lesson books level D and E for $25 each shipped for used. I have also purchased from ebay and homeschoolclassifieds. For the worksheets, I purchased the computer disk that contains levels A to E and Worksheets for the AL Abacus, so I can print all I need for all of my children. I have a laser printer, so it is low cost. Once the manipulatives are purchased, they are re-usable for all children and then can be sold when finished.
I would hesitate to switch them if they are not struggling with the program. I like that RS uses many different manipulatives and ways of explaining concepts, but with MUS they use only the MUS blocks. In RS they are learning subtraction also with the part-whole circles. There are a few lessons on subtraction near the end of level B, but they basically already know addition and subtraction if they know the number bonds that make up the part-whole circles. Then they just look for the part instead of the whole for subtraction. Example: 8 and 2 are the parts in the small circles that come together for 10 in the large circle. So if they know this, they also know that 10(the whole) – 8(the part) = 2(the other part). Wish I could draw the circles to illustrate. It could be that if your youngest is familiar with the part-whole circles that he would easily understand subtraction now, too. However, MUS is very reliant on the MUS blocks and the colors to represent numbers. They would spend a great deal of time learning the blocks at first, just as students in RS spend a great deal of time learning the AL abacus at first and seeing that five blue beads and two yellow beads is 7, without having to count.
RS starts adding verticle in the last half of level B. In fact, they are doing 4 digit vertical addition at the end of level B and get more practice with it in level C, when multiplication is introduced. They learn their multiplication facts with skip counting on the abacus and math card games. My ds has been very successful with the RS program, who had struggled with MUS and Saxon. So if you do switch, please consider at least still keeping an AL abacus and math games around to supplement if needed. RS recommends VideoText for pre-algebra after level G, geometry. But SCM recommends MUS after RS, and I have not decided yet which we’ll use.