I believe my parents furnace was 80 - 90,000 BTU. Variable speed blower - 97% efficiency - etc. About as expensive as you can get but the federal tax credit and utility rebate paid the difference between the high efficiency option and the 92% efficiency unit the HVAC guy wanted to sell them. They have a site built home with a basement. Furnace and duct work is in the basement so heat lost from the ducting is not actually lost.
Yes, mobile home units can be more expensive - just like everything mobile home or RV specific.
From your description I thought you had a ducted unit but it sounds like you don't. Sounds like the one that was in my grandmother's mobile home or the one in my parent's lake house. (Just a vertical unit that sucks air in from the bottom and blows hot air out the top located in the hallway near the middle of the home (or vice versa - there are updraft and downdraft versions) (See Below)
When I was giving prices I was talking installed for the forced air heat pump. So that $4,500 for the unit + installation + having an electrician run the 220 line to power it. DIY units generally prefilled and have prevacuumed lines with fixed hose lengths. With a forced air unit generally the lines are too long and even if the unit is prefilled you need to add more refrigerant to accommodate the extra volume from the long lines.
If it takes hours to raise your house temp to 65F in the morning with a forced air furnace a heat pump will not work for you. Not unless you keep your house much warmer at night - which will cost money.
