What is the best amount of DEX to have?

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.

You're off by a factor of 10, it's 3.6m worth.
500 meso per pill * 6 pills per hour * 10 hours per level * 120 levels = 3.6m meso.
And that's accepting your assumption of 10 hours per level, which IME is not generous at all.