I've beed build models for 10+ years now and Chambers is spot on in his reply, except for one small thing: I would never digitize any plan to be used for machine control or layout. Its just not accurate enough in my opinion. I do alot of site jobs, but recently changed jobs to a highway construction company. The site jobs depending on you tolerances, quality control and expectations you could digitize you plan for a model, I dont suggest it though. All project now days should have the Cad files accessable to the contractors, if they dont ask for them. If its a private project and the engineer is reluctant to issue the files, go to the owner and explain to them the importance of accuracy and how your company uses the accuate machine control systems to build their product. As a owner that hires a private engineering firm to design a project, the owner owns the files, and they think they dont call a lawyer.
As for highway work, it becomes a bit more complicated but much more simpler at the same time. Centerline/Baseline offset is the basic format in build a road project. you have typical sections throughout a road that can be applied to a baseline. Personally I will build a road project in steps starting with the easiest sections first, such as tangent sections were the road is mostly a typical crown. Next I will move to the radial section were the cross section is usually a superelevated surface. and lastly the transitions from tangent crown sections to superelevated sections I will usually build by hand and eye; looking for smooth transitions from crown to superelevated. Finally, most of the projects I work on now are highways so there are on and off ramps. I will build those last from the point were they are not directly tied to the mainline and basically tie those into the mainline by using the locations typical section and transitioning it to the ramp itself.
Its not as difficult as it seems, its really knowing the software your using and basic surveying/layout knowledge in understanding slopes, profiles and cross sections.
As for costs; I've been freelancing most of the years Ive been in the industry, and typical model pricing is this:
for basic site projects were the custom only wants a finish grade file with the linework, no subgrades, no roadways; just taking contour information and converting it to 3D and if there are curbs, sidewalks, building pads,ect it will run $400-500 and acre.
If there needs to be subgrades added to the file I will usually add $100 an acre (typically anyone can build from a finish grade model if the subgrades mirror the finish grades)
On highway/road it breaks down like this: a shoulder to shoulder only model will run $500 per mile/per one vertical profile (so in a case of a 4 lane highway $500x2=$1000 per mile)
For highways/road that have medians, curbs, side slopes i charge $1500 per mile ( all the median slope, curbs, and side slopes all need to be calc'ed by hand and then imputed to 3D format)
So thats a bit of breakdown of 3rd party cost, and in house cost are going to vary depending on how you or your employee's salary is setup. I can tell you though that I just built a 8 mile highway, the Eastbound only shoulder to shoulder and it took me 55+ hours just for the 3D model. I add a bunch of other line work files and such for out field guys like MOT plans, barrier wall locations, stripping lines, etc so that they dont bother with calling our surveyors to come layout something so simple. I typical add a lot of line work files and go though each phase (some highways have 3-4 phases) so that they understand were things are like barrier wall, stripping phase edge lines, etc.