I am a local Realtor in Winchester, IN. The biggest problem with the values on manufactured, as of late, has been a reluctance on the bank's part to loan on them. We have been getting some moved, but it usually takes a large down payment. As you know, our area is not full of that kind of buyer. A large portion of the manufactured sales have been on contract. Take a lot at the county assessment to get somwhat of idea on value, then compare with others that are listed. As a word of caution, watch the wording in the contract. Keep in mind that at the end of that contract is a balloon payment that you will have to be able to a loan on or you will lose everything.
Ask a local realtor to look at recent sales of modular homes to see what the value is in your area. Also, make sure to know if the modular is coming with the land or if it is a lot that has monthly rent. Owning the land makes a huge difference.
To the seller, it is. The question is, is it worth it to you, or the wider market place? Mobile homes, no matter how nice are not equal to standard construction built homes and they do not hold value the same. If you have the manufacturer and the year it was built you can find out what the home likely sold for new. Add to that the cost of site prep and land, assuming it's not in a park, and you can get some idea.
Chances are they are offering these terms because they couldn't get that price any other way. Talk to a local agent about finding other mobile homes in the area and ask them to send you some listings similar to this one. You'll be able to see what other options may be available for the same or less money.