One serious idea I had was to try to swap the lake bed rights for part of a roadway adjacent to my property.

We are no considering the partition, to have fewer lots and just build a roadway on the public easement, which is currently a gravel driveway.  

Several of us looked at expanding one back in 2001 but the cost was at least $7k per pad plus sonar tubing piers.The roadway could be just gravel but getting utilities pulled was about $800 per lot even though we would be doing a lot of the work.Finally given the remote location, cheap lot rents on the older run down MH's etc it just wasn't viable.Then a fellow investor somehow got approval for nine MH lots on his farmland, much closer to a township & he is making good coin.

One is a lake property that has additional land at the top of the hill that is seperated from the main area by a private roadway.

Something seems off, unless it was under a roadway that you had to build a temporary bypass for traffic and had engineering requirements that added a major expense.

Yes, you can have a triple net, having the tenant pay taxes and insurance and the cost of clearings and maintaining fencing, roadways as well as any improvements, buildings, well, irrigation, whatever.

Some were taken for roadway use, such as an alley to a street and abandoned.

Usually very old alleys and roadways that were never developed, chopped up by sections sold to neighboring property owners years ago.

There is a roadway that completely encircles the building.

You'll need more information for anything accurate; but from a very high level, I'd estimate roughly $50,000 per acre or $500 per lineal foot of new roadway.