Is it okay to leave a conference paper as a pre-print? Background: I just got an acceptance for a conference paper. While the conference isn't half bad (Nothing top of the domain. It's a top 80 conference in my domain as per guide2research), they are asking a hefty fee for registration and I won't be sponsored for it by my university.
Question: Is it okay to just keep the paper as a pre-print? I am extending that paper with another concept and a few more tests to submit it to a journal anyways.
In the eyes of a CS PhD (US) admit panel, does it make a difference whether I publish this paper or keep it as a pre-print?
P.S. If it makes any difference, I am an undergrad.
While @cheersmate's answer explains the difference between pre-prints and papers published in conference proceedings (however please note that none of this is necessarily true outside of Computer Science as conference culture is different between disciplines), I think it is important to mention some etiquette about publishing.
It is considered very bad form to send a paper to review, and then retract it post-acceptance. There are some exceptions to the rule (e.g. sudden illness, non-trivial errors in the results or data), such a retraction (or simply not paying the conference fees) can reflect very bad on you, as it points to:

Lack of foresight if the reason is lack of funds.
Conferences typically have publication and attendance fees. As historical fees are usually known, this is an expense that could have easily been foreseen. This is even more true during the current pandemic situation, as many conference fees are reduced to a fraction of their typical price and the conferences shifted to online events.

Lack of respect for reviewers time (if the reason is lack of funds; or lack of time to do the required revisions).
In order for a paper to get accepted to a conference, it will get assigned to an editor / session chair. That person will then need to read at least the abstract and the keywords, and attempt to solicit 2 or 3 reviewers for your paper.  They will contact more than that, and many will refuse. Finally, the editor/chair will read the reviews, and will have the responsibility of making the final decision about acceptance or rejection (it's not as simple as a ranking list where top-X get in). Indicating that you never had the intention of publishing shows a disregard for this process and the time of all the people involved.

Problems in academic conduct (if the reason is e.g. a dispute between co-authors).
It does not reflect well on anybody if you can't keep it civil with your co-authors for the period of time between submission and publication.

Problems with research rigour (if the reason is non-trivial errors in the results).
Infinitely better post-acceptance than post-publication, but... well.

If you do not give a reason, nothing good will be assumed.


My point not to make you feel bad, but that a paper that was peer-reviewed and accepted for publication, but not published (due to not paying conference fees/not presenting) does not give you any bragging rights on your CV. And while your plans to publish it in a journal certainly sound nice, a bird in the hand is worth two in the bush.
To address your current situation: the one thing worse than retracting the paper by quietly not paying the conference fees is paying the conference fees and then quietly not showing up to present your paper. That might even land you on some blacklists on larger conferences, e.g. IEEE organised. If you truly can not find funding, I would suggest informing the conference as soon as possible for your reasons for retraction, and offering your apologies for your lack of foresight, rather than just quietly not paying.
I second @cheersmate's suggestions to look if the conference can waver the fees, or has any &quot;student support&quot; mini-grants, or if something might be available through your University (or even through a lab/team/professor if you are associated to one through an internship or TAing) or your co-authors. While it would have been better to do all this before submission, I hope you still find an avenue that allows you to publish your paper.