I always wondered what are the attitude of the aliens towards money, poorness, economy and things like these.
Well, an economy doesn't have to be based around money. Money is something with a set value, so to our species it made sense to use that in lieu of barter because people were getting into cohorts over what their goods and services were worth over what they wanted in exchange. A barter system is still a form of economy, so long as an agreement of what my property is worth for what I want from you. In the case of the galactic "society" that seems to exist in the Animorphs universe there is a basic economy of trade.
The Yeerks act for the power, to enslave other races, they are no interested in money, and they have no currency, like other species.
The Yeerks learned the concept of trade after their first conquest of the Hork-Bajir homeworld. When they escaped the Yeerk homeworld, their first instinct was to take what they needed and flush any beings they couldn't infest out into space.
But once they conquered the Hork Bajir homeworld and the war with the Andalites was a reality, the Yeerks began making alliances with races they couldn't infest and who were not happy with the Andalites either.
For example, they promised Taxxons food in exchange for host bodies, which is really the most basic form of trade even for our species. In the earliest versions of our own societies people gave service for food. They served in the army, they raised farms, or if they were too sickly or injured to do one of the above they performed crafts or other services.
The Yeerks gave captured Andalite technology in exchange for a Skrit Na Moon to use as a listening outpost. So giving up one advantage for the possiblity of gaining more information was a necessary exchange to aid in the war effort.
It seems to be the same for Andalite society, and Ax in #14 says something like "I forget humans use money" (they are talking about betting on horses) implying that it is an unknown concept fot the Andalites.
I wouldn't say it's unknown. Elfangor understood the concept of money when Chapman tried to get him to use the Time Matrix but his response was, "What would I do with human money?" Implying that money is common in many races, but that Elfangor rightly points out that our currency would be as useful as straws to the Andalites. (Before the morph exchange post 52 of course)
You also have to remember that Ax is no older than the other Animorphs. So when you have a fourteen year old's understanding of the unvierse, you can't base a heck of a lot on what he says about how things work.
Well, do the Andalite have the concept of money or not? Are there rich and poor andalites? Do they have some kind of work ethic? And could money have an influence on their ability to build spacecrafts? Or they have a sort of barter system?
The Andalite economy seems to be based on the aforementioned system of exchange, wherein each family performs a service or a function that's necessary to the entire species and recieves what they need. Elfangor and Ax's family, for example, built the heating systems for ships if I recall correctly. So as a functioning member of the species they recieve proper medical care, schooling, and whatever other services are offered on the Andalite world.
So any of those Andalite families could have had things to offer in exchange for a fair amount of human currency that would allow them to function as tourists on Earth. They probably offer similar bargain to humans in exchange for their DNA to allow them to morph.
But this assertion seems to be contradicted in #38, when Aloth says he was selling organs on the battlefield, and in book #54 when Marco mentions "wealthy Andalites" that come to Earth as tourists.
Again, what's valuable to an Andalite may not register on our radars. Aloth has his own ship as well, which he could have obtained through illegal means. And he also bargained Andalite officials, probably exchanging a straight death sentance with the possibility of immunity if the suicide mission to kill Visser Three actually succeeded.
Aloth could have traded those organs in exchange for weapons, land, alien technology, and anything he would need to not have to be an unwilling member of Andalite society. Maybe he was just crazy and he exchanged the organs for hoof clippings and the Electorate didn't want that bit getting out. We'll never know of course but the point is money isn't the only thing of value in the universe.