AKA anyone bored and want to go on a fashion hunt?

So DH and I have decided to spend Christmas with his grandmother (and my ILs). My MIL made us Christmas lunch reservations at Pinehurst Resort. My FIL told me that the dress code for men is "sports coat, tie" and for women it's "the equivalent" (ugh).

Now I'm a jeans sort of girl. Like live in them. I asked my MIL for some guidance on what exactly to wear (and I think I have a decent idea as to what's expected) and she told me we'd go shopping between now and then, but I'd like to have a decent idea of what I want (or find a dress first). I thought I'd ask you all for some help--I've seen you all come up with some gorgeous clothes time and time again, and if nothing else, it's Friday afternoon, LO is asleep and I'm restless.

It needs to not only meet the standards of the resort, but also of DH's very conservative grandmother. At least knee length, (although I'm sure a little higher would be okay, but really not interested in floor-length) preferably long-sleeved since it will be winter. I have tattoos on my upper back that will probably need to be covered. I'm not opposed to black but would prefer something like a plum or dark green. Not super low cut (not that I have a ton of cleavage anyway). Preferably under $100.

Any takers?