--Tractor you purchase, (you may be able to save a lot on the stretch here)
....will you go all electric, propane, hydronic heating, etc
....how many windows and what type, (dual pane)
....insulation, (costs more than you may think)
....exterior finish (aluminum, frp?)
....slides, how many?
--your interior(will you use apartment size appliances or RV technology)
--which professionals you choose (huge variation here, will you sub out different jobs or turn it over to a company)
If you do the work yourself (or control it closely) you may be able to hit that target. The overall length will have a large effect on your costs and the garage will be a wild card.