The first mention of the daktil-dollar exchange rate comes in Aliens for Breakfast. <PERSON> mentions the Brigade's budget:
The Interspace Brigade works on a very tight budget. We have 47 million planets to look after. Our yearly allowance is 249 billion daktils. That comes to about sixty-seven cents a planet.
So at this point the exchange rate is $1 = 7907 daktil.
"Many months later" during the events of Aliens for Lunch it appears that the daktil has risen against the dollar. We first learn that <PERSON>'s new form of transport is much more cost effective:
“Microwave beaming is the latest in budget transport methods,” said <PERSON>. 489
“Much cheaper than freeze-drying. We save fifty thousand daktils—that’s $18.04 in Earth money—each trip.”
which amounts to an exchange rate of $1 = 2771 daktil, so in the course of those months the daktil's value has increased by a factor of 3.
In a later encounter in the same book:
"We used a new method—holo-faxing. Only ten thousand daktils, or three and a half Earth dollars per item."
which is a rate of $1 = 2857 daktil. This is broadly similar to the earlier number, and the discrepancy is maybe just a rounding error.
Finally we learn about the cost of the invisibility technique:
“The Brigade works on a very tight budget, young man,” said <PERSON> sternly. “Every ten minutes of vanishing costs half a million daktils—that is $183.15 in your money.
This works out to $1 = 2730 daktil, which is again broadly similar to the other values.
In the final book Aliens for Dinner I did not encounter any mention of daktils.
So it seems that in the events of the first book $1 is worth about 8000 daktil, and in the second the value of the daktil rose to about 3000 per dollar. All the values given in the second book seem roughly consistent with each other. Clearly there are a lot of daktils to the dollar, but it is not as outrageous as "a few million is worth two to three earth cents" - in fact it's not so different to the exchange rate for the Italian lire. 489