How valuable is a conference paper in Avionics and Aerospace Engineering?
There is 2 parts to this question:

(1) How valuable are conference papers to grad school admission?

The answer to that is field-dependent. In some fields, the value of conferences as means of publishing scientific content is perceived small. In others, primarily Computer Science, conferences are of major importance, hence papers in good conferences are perceived as very valuable for admission.

However, arguably, even in fields were conference papers count less, having published in and attending good conferences may make you stand out of the crowd in grad school admissions. Primarily, in these fields (e.g., physics), undergrads that have good journal papers to their name are very rare, so having been to a conference as an undergrad is already pretty good, even if it does not count for all that much in the grand scheme of things in the research field.

(2) Is it worth it given that conferences are very expensive?

I am of course generalising here, but as a rule of thumb, you should not have to pay for your own conference travels, as long as you have an accepted paper as an enrolled student. Most universities have stipends and travel grants that you can apply for, you can act as a student volunteer, or, most importantly, your university department covers your travel expenses.

Assuming that it is indeed not feasible for you to have your costs covered, you will need to take the decision yourself. Whether it's "worth it" depends not only on the value of a conference to the admission process, but also on how much >1000 USD are to you, personally. Clearly, the story is different if you are a foreign student living of a small stipend than if you come from an upper middle-class family in the US.