They might be paying a competitive _base_ salary but stock is frequently > 50% of Senior employees compensation package
"There are no plans to sell Basecamp or take the company public! But, in the slight chance of that happening, employees may be eligible to receive a portion of 10% of the value of the company if Basecamp is sold or made public. That 10% would be divided into shares, and shares distributed amongst employees based on their tenure at the time of the sale/offering."