Hmmm....I am thinking that the joke is referencing the economic concept of Demand. Demand has two parts to its definition, the first is the easy part, that you have "a want/desire for something"...second part is harder...."and the ability to pay for it."
Obviously most economists cannot afford the Ferrari and so they only fulfill the desire part... not particularly hilarious, but certainly expressing the idea.
There are a heck of a lot of people who don't fully understand what you just said. Yes, for demand to exist, there not only has to be desire, there has to be the ability to pay for. I'm glad you brought that up.
Going a little off topic, that's a large part of the reason why we are highly likely to run into more unemployment in the future if we don't have more government intervention in our economy as far as dealing with distribution of income. As we become more and more productive per hour, our productivity will eventually outstrip the growth of demand (both desire and ability to pay), and there simply will not be enough jobs for every family to have a full time job (by the way we currently define "full time").
As that happens we will either need more and more welfare, or we will suffer more and more poverty, or both. Neither are acceptable to me, and I doubt that they will be acceptable to the general public. I suspect that as we get closer to that point, hard core "cut taxes for the rich and reduce government spending" type conservatism may become far less and less popular.
Fortunately, increased poverty and increased taxes on the non-rich to support those who can't find jobs does not have to be our only alternative. We could take steps to keep demand high, things like cutting tax rates for the non-rich and taxing all income identically, instead of giving huge tax breaks to the rich (particularly in the form of capital gains and inheritance tax). And/or we could lower the 40 hour a week threshhold for overtime to spread the jobs that we do have enough so that there is at least one job for every family.
We actually saw the spreading of jobs throughout all of the 20th century, our average work week fell by about 20 hours per worker. With the exception of establishing the minimum wage, this pretty much happened naturally, through natural market forces (and with a little help from unions). However, one of the things that allowed this to happen was that our work/leisure preference changed, especially during the late '40's through the '70's. During those decades, incomes rose at all income brackets more or less evenly. Since the income for the bottom 95% or so has stagnated or even declined just a tad during the past 35 years or so, I don't know that we will continue to see this natural shift towards prefering more leisure time.
