I would think that you could wear what makes you comfortable as long as it's not flashy, or the opposite...you don't want to look like you're in mourning either!
What color are the groomsmen wearing? You can "match" (not an exact match, but you know what I mean) them...making it obvious that you're the mother of the groom...
Macy's has some beautiful things, and if you catch their sales or clearances, you can get better prices than Walmart!! Check out TJ Maxx, Ross Dress for Less, and those types of stores too...name brands at awesome prices!
No matter what you wear, I'm sure that you're going to be beautiful and appropriate! Hey! How about a quilted skirt? :lol: