One of two pseudo-scientific terms I regularly use when justifying my obsession with conlangs, and when talking generally about learning some language, is an “entry fee”. By this term I mean the amount of effort you need to master the language to some degree.
I never tried to build a theory about it, putting some levels, calculating percentages and drawing graphs basing on some vision, which in my case would probably be inacurate and biased. Just because my knowledge of theoretical linguistics is pretty bad, I tend to keep it that way. Actually, this might be someone’s master thesis - but I doubt that there were really a lot of investigations in this field. What makes any serious study complicated is the fact that perception of different things is still different for different people.
I do believe that ability to learn a language is complicaed misture of natural talent, general knowledge, number of languages the person already has in his arsenal, cultural environment, place of living, personal motivation and many other factors. However, when you fix all these factors for one person, it is obvious that in that particular situation, one new language will seem easier than the other.
When a person starts to learn a language which belongs to the same language family as his mother tongue, he might think that learning langauges is easy. Very quickly this person starts to realize that “false friends” are his greatest enemies, but this does not stop him from having fun of learning. If the person selects a language from a different family, especially if its written form uses some nonfamiliar writing system, the effort needed to get even the lowest level of mastering this language is great.
As I said, I compare this difference in amount of work to be done to an “entry fee”. Imagine that you are starting to do some sports. If it’s jogging, your investment is a pair of good running shoes - everything else is an option. If it’s a yachting, you need to buy or rent the boat, which seems to require much more money and time. The same with languages - entry fee in the francophone world is much smaller for catalans than for chinese or russian, while Polish will be much “cheaper” for slovaks rather than moroccans or mexican.
If you learn several languages in a row, you get a “discount” for every next language you start learning. If you know Norwegian, you get huge discounts for learning Danish. If you ever spoke Russian, the price of learning Ukrainian is much lower than if you never did.
What’s interesting in this theory in application to conlangs is that the latter ones were often designed to be easily learnable by people with different language backgrounds. There are some great interlinguistical projects that specifically aim to be “a missing link” for some language group, like Interlingua for Romance languages or Slovio for Slavic languages. Having them learned gives great discounts for picking up related natural languages. Pidgins and creoles do the same job, as they often mix two or more natural languages in an easy package.
Using obvious similarities between the languages, you might build a learning path for complex natural languages. My favorite example of such path can be illustrated by an idea once spoken by Dave MacLeod - to learn Arabic, you might at first learn Interlingua, which helps you to pick up Italian and Maltese which mixes good portion of Italian being at the same time a language family as Arabic. Going from Maltese to Arabic is maybe a longest part of this path, but it’s definitely shorter than going straight to Arabic. You might spend the same time to learn Arabic from scratch as if going the Interlingua-Italian-Maltese-Arabic path but in the latter case you get 3 more languages, which will also give discounts for entering the world of other languages. Think about it as if you buy one thing, or getting it for free while buying three more useful things that in total cost the same. Or rather getting three things for free for the same money. Isn’t it just great?