it depends. i've been hoarding neon runebooks for 3 years and i have about 200 total... most are the "common" colors though - aqua blue, dark blue, lime green and bright pink. i always love finding the more rare colors like bright yellow, bright orange and "velvet-rope red". the sigil purple ones are fairly rare and very pretty. i usually buy them (all colored neon books) for between 10k and 200k depending on color and availability. it's really hard to find neon books on some shards like baja and pacific... and really easy to find them on shards like chessie, europa and atlantic. hope that helps...
oh - also, people do still have neon runebook dye tubs, which is why the more common colors are so common. there's also a really pretty soft green that is becoming easier to find (it's a very blue-green, not a yellow-green) - i'm assuming it just changed hands into someone who is using it a lot more than it's former owner to dye runebooks soft neon green.