Spamming roads everywhere is, in fact, completely realistic. Anywhere on this planet with a city anywhere near it is positively crowded with roads leading everywhere (and, occasionally, nowhere). Based on the real earth you should not only have routes on all such plots but be able to build multiple copies of each type of route on them (making it take longer to destroy all the routes).
Even on the largest maps that represent an entire planet, a single plot is about 100 miles across in both directions for equatorial plots (away from the equator the E-W distance should shrink, but doesn't), and it is larger for smaller maps. Good luck finding very many places on the planet Earth that measures 100 miles by 100 miles that do not have some sort of road (or, at least, dirt path) or railroad in it. Antarctica, central Greenland and some other arctic locations, and probably some large deserts and a few large regions of jungle with lots of waterways are the main places you could find such locations (most of them have few people living there, some of them have none). Other than that, they are very rare.
As for cost, roads that connect places to other places (like the places your food and other things come from) pay for themselves. The increased commercial activity more than covers the cost. If this were not true, you would not be able to find them all over the place in the real world.
As long as we are talking "realistic", you should not be able to work (at all) a plot that is not connected to the city via a route of some sort (rivers count as routes for this). Hauling tons of broccoli (or whatever it is that the farm 2 plots away from your city is growing that has no specific resource) across 100 miles or more of roadless and riverless terrain just does not happen, except in the modern world where stuff can (but mostly isn't, except in an emergency) be moved by air. From a game play perspective this would make pillaging the plots around an enemy city more effective.
