@DaCubz;
Sorry for the delay, I was sick for a couple days. Well, infantry/mg costs. In "real" terms, what we need to do is figure out what volume of manpower England was able to send into France after the various bloodbaths, and from that determine what the needed "training" time was for a given number of men- assuming of course that it is possible to translate a single 'unit' in Civ into a given number of soldiers. England committed 5,400,000 soldiers to Flanders & France over the 4 years of the war.
In practice, if the men were needed, their training would just be shortened of course. So, it depends upon how "realistic" you feel the scenario should be. &, I'm just using England as a test case because it is relatively easy to judge her production values, as well as the fact that she should be able to be the pre-eminent naval power, at least until such time as America decides to invest her production in surpassing her.
I suppose if 4 of her 8 cities are building ships for ~15 months, the other 4 should be able to build units at a sufficient rate to compensate for the units the other cities would be raising, since a Real city could "build" more than one thing at a time. So, whatever the "rate/cost" of infantry turns out to be, it should likely be discounted by up to 50% to compensate.
The downside is if you had an England that did not Want to build ships, she could raise twice as much manpower as she otherwise could have. Many pros & cons to consider.