Why not? I'm actually curious and would like to know more of your reasoning because I've thought it the other way around. For me, mechanical keyboards may have a large price but spending the money to take the step into mechanical from rubber dome was a HUGE difference and well worth the price. The feel is COMPLETELY different. Mice on the other hand, I've felt that as long as it fit my hand and didn't have a crappy dpi that they had very little benefits from more expensive ones and usually had the same feel(accuracy).
My flat-mate has a Logitech G17 keyboard and two of my mates have the Corsair K60 ones, my gaming is by no means enhanced by using their keyboards compared to my 20£ Logitech poop-board. Yes the built quality is better, but that's about it as far as I'm concerned.
Mice on the other hand:
Battery powered mice (be they gaming grade or non) are completely out of the question - They always run flat at the most annoying moments, they often have latency issues and they are WAY too heavy.
But gaming grade mice in general, have so much better precision that it really makes my accuracy in BF3 for instance, around 50% better than with cheap ones. You have extra buttons for options like; meele attack, reload, autosprint, prone etc. and they don't break down in 6 months like regular cheap mice do when used a lot.
I've learned to stay away from Razer mice, as I find their products nice enough but their build quality way below standard. My current logitech G5 has served me for 3 years now, and I play computer at least 40 hours per week. No cheap generic mouse can stand that, and still feel like it's brand new.
Switching DPI on the fly is really a must for me, cause I tend to play many different classes, and turrets in BF3 respond so slowly.
No ordinary mouse will let you do that, and cheap gaming mice often don't do it well.
This is all just my personal opinion, but I've been gaming on the pc platform for about 20 years now, so I've tried a lot of hardware.