[vgm]

The cost is negligible with respect to the total carrying capacity of your brain. I don't have a source, but I've seen this stated in a few books.

It is often mentioned that there is a cost in that an additional association seems like more work. (To disclose my bias, I find visual memory techniques to be a boon to my apparently concept-driven mind.) Consider, however, that work must be done both when constructing the memory and also when repeating it. Visual memory, mnemonics, and the like, all work, I believe, because they increases the strength of the encoding, decreasing the need for repetition later. So, there may be more work upfront, but there is much less over the lifetime of the memory.