I don't see why Mek Gunz would sell fewer models than Wave Serpents. You can run five Mek Gunz in one slot, and they did their best to try to make you buy multiples if you want to swap weapons (you can fudge it, granted). You're not going to have many more than five Wave Serpents in an army. The boxes aren't as big as the Wave Serpent, and I doubt the molds are that much more complex (if at all).
For me, I'm sticking with my older artillery. People have no problem with me using Pulsa Rokkits as "Bubble Chukkas," for example. If I need others, I'll convert them. But heck, they stuffed Heavy Support so tight that it's often hard to find room for even one gun battery (with the new Orkanauts, Lootas now in there, Flash Gitz, Battlewagons, Looted Wagons, Deff Dreads, and Killer Kanz).