I really have no idea how much mk4 mini will sell for (plus Dr. Neo hasn't announced all the specifications yet, so we don't know what it will be able to do). The main reason I think mk4 mini might be more affordable is that the neoflash products have become less expensive in general, since the first generation product was introduced (that was well over $200 US dollars). MK3 2006 is around $90 or so...
Depending on what features it supports and your particular needs, you might still need a separate flashcart. Mk4 mini mentions flashcard support, so that would indicate to me that it may have more limited functionality (it's all conjecture, until more information is released)
You may already know this, but the neo-key is not a good substitute for a regular gba flashcart... The neo-key has only 16 M writeable space, and NO sram at all...
There will be more than one variation of mk4 (this is probably the 'lite' version)
We just have to wait and see (more information should be coming soon, according to Dr. Neo's post). The mods really don't know anything more than everyone else at this point.
BruceG