Construction Bids for basement

[Randall Owens]

BPs community,

So I have had four contractors come out and do bids on a basement that I am  planning to convert into a legal third unit. The bids have been very inconsistent with numbers ranging as low as 45k to as high as a 100k. At this point, I am trying to figure out if the contractors offering bids between 80k and 100k are overpricing their job and trying to finesse me. What I did notice is that the contractors that were charging larger bids were those that had bigger contracting companies and the ones with the smaller rates was not as big of a contracting company but still related they could get the job done at a much cheaper price. At this point, I am not looking for an extravagant finish in the basement just an additional unit that is nice, comfortable, and livable.  Any advice?