If you are interested you can use
this reference sheet to help you match CIMs to all the different gearboxes taht they can mesh with. It is slightly out of date as newer products have come out. But looking at it tells me that the AndyMark Planetary Gearbox can do roughly what you need, though the price isn't going to be much cheaper. Same with the VersaPlanetary.
Your best bet might be to go with the
Vex Clamping Gearbox with Vex plastic bearing block (as shown on the page) and to play with the reduction that you plan on using. It will be a bit more complicated and isn't quite as versatile to mount, but if your main goal is cost reduction then this would be a good fit. I haven't added the cost of those individual items together but I'm estimating that you will still be in the $80 - $90 range approximately.