If it is for an assignment, then students ideally must not be expected to pay $50 to obtain the software for themselves just to complete an assignment.
Best Case Scenario
The University will usually have a license for the said software. You could possibly request and urge your professor to get a license. Here are the pricing details on the Spartan page.
Once you have a university license, you can use the university license server when prompted for the name of the Spartan license server.
Here are detailed instructions for a student of University of California San Diego (UCSD). So you can follow the instructions and change as per your university license server.
Worst Case Scenario:
In the worst case, that you are still expected to purchase this software, and you are not in a position to buy it, then you can utilise other alternative free open source tools such as Python to do this:
Here is an article which clearly explains how to calculate eLumo and eHomo using various python libraries. Both are free and open source.