Actually if the utilities are included in the $100, then you multiply by 60, not 70. If utilities are included, then the lot rent is less profitable, so it is ‘only’ worth a 60x multiple, not 70. But if the utilities are billed in addition, then you multiply the lot rent by 70.
You then add in the value of the homes. By ‘value’ we mean what the homes can be sold for on terms in the park. You’ll basically want to sell them at cost, help others become homeowners, and keep the lot rent for yourself. Frank & Dave’s books go into this more, but a rough rule of thumb is that old 1970’s mobile homes in decent condition might be worth $2,000 - $4,000. Newer 1990s mobile homes might be worth $10,000 - $15,000. But one does not cap the income from mobile homes. They are just worth what they can be sold for in the park. So you should inspect each home, and run a few test ads and really see what they are worth to your market.
Frank & Dave generally stay away from parks with really low lot rents (anything under $150/month or even under $200/month). Low lot rents are generally a sign of a poor economy. Many of the $80/month - $150/month lot rent parks are in the southeast, which has a perpetually weaker economy than the midwest or either coast.
Frank and Dave’s books will step you through the due diligence you need to do, but the only way I’d buy a $100/month park is if the park was in good physical condition, the surrounding economy was strong, and every nearby park had $200/month lot rents. That would indicate the rent was mispriced, rather than a reflection of the surrounding economy.