I don’t think you could do it from the metal frame up and beat the costs of building it in a factory. But for $15 - 20k you can take a real junker and make it pretty much all new inside and vinyl side it, esp. if you are running a crew yourself. Thing is, is it worth it? If it has a pitched roof, I would say yes in most cases. If it is a little old flat top, I would guess not. Of course if you do a complete makeover on the a home you don’t have the cost of trashing it out.
One of the questions to ask yourself is how much would you be willing to loose to have a home in your park. Most owners would be willing to pay a $4k moving fee to have someone move in. What about $5k? Or $6K? If you take these numbers, say $5k you may be willing to loose on a deal, plus $3k it may cost you to make a junker disappear, then spending $20k to turn in it into a dollhouse is not so bad. Surely you could sell it for at least $12k, if it is all shinny and new , inside and out.