Question: I wrote a paper with 3 other students regarding my final year project in mechanical engineering. It was recently selected for a conference in NIT Delhi. Minimum one author is required to present and the price is pretty steep for the participation certificate.Should all of us attend the conference or just one of us? Does the certificate hold any value?

Answer: 
Perhaps things are different in India than some other places, but I'd suggest that at least one of you go, as required. The others should make a decision whether the conference, itself, independent of the presentation, has enough value to justify the cost in money and time spent. If any of you intend an academic career, the opportunity to meet others has positive value in building future circles of collaboration.

Note that there may be some discounts for student presenters.

Ii think that, in general, a "certificate of participation" has little value other than as a remembrance.

Of course, if you can draw on a grant for this, the computation changes quite a lot, and then the main cost is the time and effort for the individual. That may not be trivial, of course. Talk too your professor(s).

And, congratulations.
You should pay as little as possible to a conference. If there is only one presentation, there should only be one paying presenter.

For online conferences, you may be able to submit your presentation as a recording. The people who appear in the recorded presentation need not be the the paying presenter, unless the conference says otherwise; all the authors could be in the video. Do follow pandemic quarantine rules.