8. Share-Based Compensation

We issue stock based compensation awards under the Plan. The purpose of the Plan is to advance our interests and those of our shareholders by providing incentives to certain persons who contribute significantly to our strategic and long-term performance objectives and growth. An aggregate of not more than 3 million of our Class A common shares, subject to certain adjustments, may be issued under the Plan, and the Plan terminates no later than April 23, 2019. The exercise price for our options is generally equal to the closing market price of our stock at the date of the grant, and the options normally vest at 2% per month for the 50 months beginning in the eleventh month after the grant date. Follow on option grants begin vesting in the first month after grant. We recognize the compensation expense related to share-based payment awards on a straight-line basis over the requisite service periods of the awards, which are generally five years for employees, and five years for board members.

The determination of the estimated fair value of share-based payment awards on the date of grant using the Black-Scholes option-pricing model is affected by our stock price as well as assumptions regarding a number of complex and subjective variables. We derive the expected terms from the historical behavior of participant groupings. We base expected volatilities on the historical volatility of our stock over the expected term. Our use of historical volatilities is based upon the expectation that future volatility over the expected term is not likely to differ significantly from historical results. We base the risk-free interest rate used in the option valuation model on U.S. Treasury zero-coupon issues with remaining terms similar to the expected term on the options. We primarily use historical data by participant groupings to estimate option forfeitures and record share-based compensation expense only for those awards that are expected to vest.

The following are the variables we used in the Black-Scholes option pricing model to determine the estimated grant date fair value for options granted under the Plan for each of the years presented:

In 2015, we commenced issuing restricted stock units (RSUs) under the Plan. The RSUs entitle the recipient to receive one share of Class A common stock for each RSU upon vesting. The RSUs vest with cliff vesting in 5 years, provided that the recipient is still an employee or director of Gaia on such date. The RSUs will be automatically forfeited and of no further force and effect if the vesting conditions are not met.

We use intrinsic valuation for RSUs, which due to the nature of these awards, is typically market price of our common stock on the date of grant.

The table below presents a summary of activity under the Plan, as of December 31, 2017, and changes during the year then ended:

The table below presents our valuation data for the Plan:

We issue new shares upon the exercise of options and vesting of RSUs, which will occur in 2020 and 2022. We received approximately $0.1 million, $1.0 million and $0.2 million in cash from stock options exercised during 2017, 2016 and 2015, respectively. The total intrinsic value of options exercised during 2017, 2016 and 2015 was $0.1 million, $1.8 million, and $0.1 million, respectively. The total fair value of options vested was $0.3 million, $1.3 million, and $0.9 million during 2017, 2016 and 2015, respectively.

As of December 31, 2017, there was $5.9 million of unrecognized cost related to non-vested shared-based compensation arrangements granted under the Plan. We expect that cost to be recognized over a weighted-average period of 3.54 years.