What is the best amount of DEX to have?

I have 50 dex on my Sader and I'm pretty happy with it.

You won't do as high as damage as dexless saders, but you save a ton of money. No need to buy accuracy pills, no need to spend hundreds of millions on accuracy scrolls or scrolled items. I do have a few accuracy items (shoes and Z-helm) but when I do get a few spare mesos, I look for str and atk based items.

Accuracy pills are cheap.

Assuming you spend 10 hours per level from 1-120 (which is a really generous estimate), you use up 7200 sniper pills, or 360k worth. From 1-120. It's the cheapest +12.5 STR you'll ever get.