The Company has the following three stock option plans that provide for the award of stock options and premium-priced stock options to selected employees, directors, and officers of the Company: a) Amended and Restated Stock Option Plan, b) IronPlanet 1999 Stock Plan, and c) IronPlanet 2015 Stock Plan.

Stock option activity for the three months ended March 31, 2022 is presented below:

The Company uses the Black Scholes option pricing model to fair value stock options. Significant assumptions used to estimate the fair value of stock options granted during the three months ended March 31, 2022 and 2021 are presented in the following table on a weighted average basis:

At March 31, 2022, the unrecognized stock-based compensation cost related to the non-vested stock options was $13,210,000, which is expected to be recognized over a weighted average period of 2.6 years.

The Company also grants premium-priced stock options to the senior executives with exercise prices above the fair market value of the Company’s common shares on grant dates. The premium-priced stock options vest and become exercisable upon the third anniversary of their grant date and expire on the sixth anniversary of their grant date. The fair values of the premium-priced stock options were calculated on the grant date using a Monte Carlo simulation model. There were no premium-priced stock options granted during the three month period ended March 31, 2022 and 2021.

At March 31, 2022, the unrecognized stock-based compensation cost related to the premium-priced stock options was $7,063,000, which is expected to be recognized over a weighted average period of 2.5 years.

Share unit activity for the three months ended March 31, 2022 is presented below:

The total market value of liability-classified share units vested and released during the first three months of 2022 was nil (December 31, 2021: nil).

Senior executive and employee PSU plans

The Company grants PSUs under a senior executive PSU plan and an employee PSU plan (the “PSU Plans”). Under the PSU Plans, the number of PSUs that vest is conditional upon specified market, service, and/or performance vesting conditions being met. The PSU Plans allow the Company to choose whether to settle the awards in cash or in shares. The Company intends to settle by issuance of shares. With respect to settling in shares, the Company has the option to either (i) arrange for the purchase of shares on the open market on the employee’s behalf based on the cash value that otherwise would be delivered, or (ii) issue a number of shares equal to the number of units that vest.

Fair values of equity-classified PSUs are estimated on grant date using the market close price of the Company's common shares listed on the New York Stock Exchange, as these are not subject to market vesting conditions.

At March 31, 2022, the unrecognized share unit expense related to equity-classified PSU’s was $17,735,000, which is expected to be recognized over a weighted average period of 2.2 years.

20. Share-based payments (continued)

PSUs with market conditions

The Company also grants PSUs to senior executives with a market condition where vesting is conditional upon the total stockholder return performance of the Company’s stock relative to the performance of a peer group over a three year performance period from the date of grant.

At March 31, 2022, the unrecognized share unit expense related to equity-classified PSUs with market conditions was $4,555,000, which is expected to be recognized over a weighted average period of 2.4 years.

RSUs

The Company has RSU plans that are equity-settled and not subject to market vesting conditions.

Fair values of RSUs are estimated on grant date using the market close price of the Company's common shares listed on the New York Stock Exchange.

At March 31, 2022, the unrecognized share unit expense related to equity-classified RSUs was $3,636,000, which is expected to be recognized over a weighted average period of 1.8 years.

DSUs

The Company has DSU plans that are cash-settled and not subject to market vesting conditions.

Fair values of DSUs are estimated on grant date and at each reporting date using the market close price of the Company’s common shares listed on the New York Stock Exchange. DSUs are granted under the DSU plan to members of the Board of Directors. There is no unrecognized share unit expense related to liability-classified DSUs as they vest immediately and are expensed upon grant.

At March 31, 2022, the Company had a total share unit liability of $9,596,000 (December 31, 2021: $10,056,000) in respect of share units under the DSU plans.

Employee share purchase plan

The Company has an employee share purchase plan that allows all employees that have completed two months of service to contribute funds to purchase common shares at the current market value at the time of share purchase. Employees may contribute up to 4% of their salary. The Company will match between 50% and 100% of the employee’s contributions, depending on the employee’s length of service with the Company.