Most of the mini cakes and mini wedding cakes start around $35 and max out around $65.
Something like this mini wedding cake would likely go for around $35.
Probably the same $35 price point for this mini cake with the mini castle...
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Now, these are more basic mini cakes...but I've seen other colors, elements, and piping added to make it a bit more unique and still stay in this range.
As soon as you add fondant, you're going to see the price rise to the $50 mark or above.
As for the GF vs. the Contemporary...there are pros and cons to both, but ultimately both are usually decent. They supposedly all use the same recipes/ingredients for their cakes- so in theory- it should all taste the same no matter which WDW bakery you use. I could give you a long list of my pros and cons for each, but my best advice (since both have wowed people and disappointed people) is to go with the one that works best for you and your plans for your mom.