Lol at $30k
If you get the chance to attend a wedding in Charleston, SC, I would definitely recommend attending. It'll cost much, much more than $30k.
Our wedding was in Charleston, but my SiL was in the wedding industry there as a planner and my wife helped her out with the big money weddings while in school. Got a lot of discounts on linens, food, booze etc as they were in the industry. No idea what our wedding cost, but suspect if was North of $50k in 2014.
Some of the weddings my SiL was doing around that time for folks from NYC and the like at Kiawah and various places in the Charleston area were $500k +.
The best advice I have for you OP, is to do what your fiance and her family say, stay out of it, and show up. If that means elope, go for it. If that means putting on a Tuxedo or a suit, then do it. The wedding ain't about you.
In terms of wedding attire, I always tell folks I'd rather look like James Bond (tux) vs an attorney/banker (suit).
****Forgot to add this:
The biggest factor in all of this is your religious beliefs. If you've got none, then carry on. However, we believe that marriage is a serious Covenant between the couple and God. That seems to be forgotten these days and I suspect as to why divorce rates are so high.