Originally Posted by Win
You really need to be, or become, informed on land values and construction costs in your area.............
All good points that should be investigated as part of doing one's due diligence in researching the property.
Realtors, like any trade, have their good agents and bad agents, so, just like any other service you would need in life, you need to find someone who knows how to do their job well, knows the locale you're searching in, and keeps YOUR best interests at heart. All too many are happy to misguide you, just to make a sale and get their commission. Dirty little secret, most agents don't know how to do property comparisons. Just like Google, it matters what you put into the search bar.
You mention that the roof will need to be replaced. IIRC, you said it's a 20 year old structure, so needing a new roof sounds about right. Have a home inspection done in addition to a pest report. A home inspection will call attention to things that are out of the ordinary or likely to become problems in the near future. A pest inspection will tell you if there are any active pests or dry rot that needs to be addressed. I would expect to find a lot of failed caulking, probably some moisture bulged siding and trim. Do all the appurtenances (hvac, water heater, propane/wood stove, etc.) have the appropriate trailer certification sticker on them, if not, your insurer isn't going to like that. It's things like this that you need to be wary of before purchase. If you know going in and don't care, that's your business, if there's a problem and you didn't know about it, that is an issue
As mentioned by others, well, septic, municipal connections, etc., and their state of repair also factor into property value. For instance, if the sewer line is collapsed and you have to replace it, you're now sitting on a $100K property that is only worth, maybe, what they're asking for it.
The property itself, are there any land use restrictions, water rights (for or against you), etc. Was there a toxic waste site on or near the property that has affected the water or land? Research at the county records will tell you these things.
Personally, what do you need from the property? Does it fit those needs? Good schools, good neighbors, what-have-you.