6:17 AM, Thursday March 17th 2022
It depends on a few factors.
If you plan on going the official critique route, you are required to use fineliner pen for Lesson 2-5 and the 250 Cylinder Challenge. Ballpoint is allowed (and even recommended) for later lessons like 25 Wheel Challenge, Lesson 6 and Lesson 7. Per this article: https://drawabox.com/lesson/0/4 - you're also allowed to use ballpoint for L1 and 250 Box Challenge.
If you don't plan to ask for official critique and only rely on community feedback then you can use whatever tools you have at hand. In this case ball point pen is fine.
Just keep in mind that there is a reason why fineliners are highly recommended in completing the majority of DAB. I do understand that you have financials constraints though so ultimately do what works for you. I think I went through 4-5 fineliners from L1 to L5 + 250 Box and Cylinder Challengers.