underestimate

an estimation that is too low; an estimate that is less than the true or actual value

assign too low a value to; "Don't underestimate the value of this heirloom-you may sell it at a good price"

make a deliberately low estimate; "The construction company wanted the contract badly and lowballed"

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"