Is $24.01 million for 15 used A320 reasonable?
Short Answer: If the 24 Million was for just 15 airworthy A320s, then yes, its a steal because $24 Million is barely enough to buy 2 or 3 A320 engines. However, Niki Airline likely has hundreds of Millions of dollars in debt that British Airways is also "buying", that is why the final price seems so low.

Longer Answer:
There is so much that goes into the value of an aircraft that this cannot be definitively answered. Even a properly maintained aircraft may be of little value if it has too many cycles (landings and take-offs) and total hours. For example, if those 15 A320s all need engine overhauls then its going to cost a lot more than $24 Million to get those done, not to mention the downtime. 

Additionally, and adding even more uncertainty to your question (sorry), is that the $24M was for the airline. As you probably know, the airline includes other assets, such as the landing rights that you mentioned, but it also includes other liabilities, such as debts to previous creditors (i.e. vendors, debt service, etc). 

So, just as a simple illustration, lets say that the 15 A320 are collectively worth 150M and the landing rights are worth 200M for a total combined value of $ 350M, if the company has liabilities (debts) of 300M, then the net value would be 50M for the whole company, and paying 24M would be a steal!  But on the other hand if the company had liabilities of 400M then the company would be worth -50M. Usually, when a company is insolvent it is in this situation where it may have some valuable assets, but it has more liabilities than the assets or cash flow it generates. British Airways is likely hoping to streamline the business and turn it around, or just scrap it and take the landing rights for its own fleet.