One of the problems with CC NC is that when you say "I am not sure what "Commercial purposes" are", you are not alone in that.
The relevant clause in CC BY-NC 3.0 reads
You may not exercise any of the rights granted to You in Section 3 above in any manner that is primarily intended for or directed toward commercial advantage or private monetary compensation.
The definition of NonCommercial in CC BY-NC 4.0 is
NonCommercial means not primarily intended for or directed towards commercial advantage or monetary compensation
What that means exactly is not very clear, and opinions differ on whether something is commercial use or not - and a whole lot of discussion about it has not produced much, if any, clarification on the matter.
Creative Commons commissioned a study to what exactly the NC clause in their license means in 2008, see https://wiki.creativecommons.org/wiki/Defining_Noncommercial and http://creativecommons.org/weblog/entry/17127 which found that opinions on what exactly is commercial use differ significantly.
For example, it is generally thought that use of CC NC materials is not allowed on websites that are supported by advertisements, even if those advertisements are only intended to cover the running cost of the website. The study cited above found that
On a scale of 1-100 where 1 is “definitely noncommercial” and 100 is “definitely commercial” creators and users (84.6 and 82.6, respectively) both rate uses in connection with online advertising generally as “commercial.” However, more specific use cases revealed that many interpretations are fact-specific. For example, creators and users gave the specific use case “not-for-profit organization uses work on its site, organization makes enough money from ads to cover hosting costs” ratings of 59.2 and 71.7, respectively.
When you want to sell it, or receive a compensation for the materials (and power) used, one of the findings of the report is that
virtually all creators agree that a noncommercial use is one in which “no money changes hands.”
When looking at the question where "money would be made from the work, but only enough to cover the costs", which seems to be your scenario, roughly 60% of users considers this "definitely commercial" (about 10% "definitely noncommercial, and the rest "can't say"), and about 45% of creators considers it "definitely commercial" (vs ~20% definitely non-commercial and the rest "can't say")
If the copyright holder believes your use is commercial, it's up to the court to decide whether it is or not, creating a legal hazard.
You can always contact the owner to make a separate deal that is not covered by the license, or discuss (and put in writing) whether he considersthey consider your use commercial purposes.
