128MB on a Mac II Series is only using 16MB SIMMs on 8 slots. 16MB SIMM, though not cheap, can be found at low cost on sites like Ebay. Compared to 128MB PC100/133 DIMMs? It's about the same if not a bit more. All one has to do is search for it.

Though I doubt I would be pushing a Mac to that extreme, more memory is always better. I do deal with multi-media files and no matter the system used, the files take up a lot of RAM to process.