Often times when I hear people talk about the cost of various things, I notice that they have a very different understanding of what the terms "expensiveness" and "affordability" mean.
To me, expensiveness describes the higher cost of something relative to the cost of other things, especially things of the same category. This could also be the higher cost of something relative to its cost in a different time or a different place. For example, the cost of an one-bedroom apartment is more expensive now than 10 years ago and the cost of an one-bedroom apartment in Vancouver is more expensive than one in Barcelona.
Affordability is the ability for someone to purchase something, which has more to do with the amount of money they have. To illustrate, an apartment in Vancouver may be expensive but it is still affordable to those who have "a lot" of money.
To really drive home my point, I offer this scenario. I can able to buy a candy bar every time I go to a store. It is affordable to me. For someone who is struggling with rent every month, the extra cumulative cost of those candy bars may prove to be too much; they could be unaffordable to that person. It wouldn't make sense to say that the candy bar is expensive simply because a person in that situation couldn't affordable. The price is too low. However, compared to the cost of candy bars in certain other parts of the world or their cost 50 years ago, it would make sense to say that the candy bar is expensive.
Finally, whether or not a person wants to purchase something is a whole other thing. Expensiveness and affordability are, however, factors that people probably benefit from considering when they are deciding if they want to spend their money on something. Is the more expensive shirt worth it to you? Is living in a place with a more expensive cost of living worth it to you? Is it really affordable for you to buy a "cheap" item, like when something is on sale or as a part of some special deal?