underestimate
underrate
make too low an estimate of
he underestimated the work that went into the renovation
Don't underestimate the danger of such a raft trip on this river
lowball
make a deliberately low estimate
The construction company wanted the contract badly and lowballed
undervalue
assign too low a value to
Don't underestimate the value of this heirloom-you may sell it at a good price
underestimation
underrating
underreckoning
an estimation that is too low; an estimate that is less than the true or actual value