Rereading the first chapter, he spent one night with her and he's already on the hook for 2 million, which I hope wasn't merely a handjob. 😅
Although if she saves his life (likely) and he already got at least a handsy from her, then yeah it was worth it. Maybe.
Because all this mess started when he decided he needed a slave assistant for what, more profit? From what I can tell, he was doing pretty well (financially, and not attracting attention from dangerous people) before he bought the two girls.
The elf slave was just a show of charity so the reader could have an example of "what a pure and good soul our MC has!".
Eventually she'll also serve as a larger plot device than she already has.
Purchasing Nora or any regular mundane slave he would have obtained would have likely been because:
A) He's lonely because he is friendless and trying to hide his abilities in an unknown world. Slaves are forced to keep secrets.
B) He is in a world with much lower tech so the amount of time and effort invested in mundane tasks such as cooking, eating, cleaning, and bathing started to annoy and burden him.
C) Even if the slave directly wasn't useful for alchemy (and just about anyone could be used as a mule for supplies/finished products), the time and energy he would save by having a slave take care of most of the daily chores above would make him a lot more efficient and likely much more profitable over time.
The only issue is either Nora abused the slave debt system or the MC was so grateful that any slave would almost instantly turn the MC into their debt slave (too lazy to reread chapter 1).