How locked into a mobile home are you?
The reason I ask is, like you, I almost knocked down an old home I owned, went looking at doublewides, and almost pulled the trigger on one. THEN I did some checking. There is a host of reasons why they are not such a great deal, from resale value, to depreciation, to bank financing (for the next buyer) ten years down the road.
But in the meantime I did some reconfiguring on the old house, and what we originally thought would cost $12,500 to rebuild, has actually only cost $1800. That was just a bonus.
Being an old house we are Grandfathered on everything, no permits needed here to replace what we need. As long as I am within the foundation of what used to be here, I am okay. That includes leach fields for the septic, an additional house site, a barn, a couple of sheds, etc. This is Maine, and yet it has saved me a lot of money. I can imagine in Oregon it would save you even more money.
BUT I have not seen the house either. Maybe it is a goner...not all houses can be saved. Some can though, but most builders today scoff at such a thing. How do you tell...look at the roofline. If it is sagging, the foundation and framing is rotted and the house is probably beyond repair. If it is straight, I might rethink knocking down the building and putting in a mobile home.
(To
answer your question; here in maine a septic system is between $7000-$15,000 dollars. A new drilled well will be around $6500).