Turning a SFH into a duplex- estimating cost

[Chelsea Mastin]

I hate to ask for help, but am having one heck of a time figuring out an estimated cost for renovation on a property I will be closing on at the end of the month. It's 1,344 finished single family manufactured house built 2005 over a 1,344 sqft basement with 9' ceilings (2,688 total sqft.) The pluming is roughly in as well as loads of building materials. The problem is that it is Aurora, CO- one block outside of Denver and all licensed contractors are booked out for months and charging what I consider outrageous prices per books/online estimates I've came across. 

The previous owner was passed away in beginning phases of turning it into a duplex; its is zoned R2. The city does not have any duplex related permits. I would like a separation of gas and electric, framing, dry wall and doors. I can then complete the rest myself and/or sub out the remaining work. I've contacted 9 licensed contractors and have only received two estimate of 30k-50k or 80k-100k for full reno. One said the pricing it's due to the height of the ceilings (9') and that since it's below manufactured home it will require "specialty" framing work. 

I would think that the materials, age or the property (2005) and rough pluming would save me quite a bit?? lol I'll get multiple itemized estimates after closing, but would like to get finances in order prior. Any insight?? These guys just taking me for a ride?