My guess is you'd only need to expand the leach field and possibly plan to have the tank pumped a little more frequently, but in all honesty, if everyone in the house hold understands the do's and don'ts of what to flush and what NOT to flush, the tank shouldn't need pumped very often because a healthy environment in the tank and it will take many years for it to fill with solids. SO, no cooking grease, oils, use only septic safe detergents for dishes and laundry, don't use the garbage disposal for anything, put those things in the trash instead, no coffee grinds, egg shells, woman hygiene products, etc. Septic systems aren't like city sewer systems, so just make sure everyone understands that and the tank should function just fine for many years without pumping.
The cost of a leech field can be all over the map for several reasons. Size, depth, types of soils encountered, etc.
I'll try to remember to check what our septic system cost us to install last year out there in Elizabeth.