Lets go first with 100 millions $, then $10 a coin. After that we can go higher.
Both of those things already happened earlier today.
What's next?
Next thing to happen is actual use in dark markets - that starts Sept 1.
https://www.reddit.com/r/Monero/comments/4z0zih/official_alphabay_dnm_to_accept_xmr_from/After that, Ring CT gets introduced in the next version of Monero so maybe 6 months?
After that, an easy to use GUI.
Price will continue to increase as knowledge becomes more widespread.