I've been saving up for something for quite some time, and today I bought it. It's a handcrafted item. The crafter sells several different ones on her website, and one was "custom design" - she will make whatever you specifically want. I chose this option though was surprised it was no more expensive than the others.
I sent across the description of what I wanted her to make, and she replied saying that I would need to choose from the available selection. I replied back saying that I thought I could choose whatever I wanted, as that's what it says in the description. It turns out that she'd put the wrong price on the "custom" item and it should be a lot more expensive than the other items. She was very nice about it, and once she realised her mistake said that she would honour it. I feel a bit terrible - thinking about it, it was obvious that a custom design should cost more. I haven't replied to her yet. I feel I should either pay more for what I really want (though I'm not exactly sure how much it should have been), or choose one of the others.
YABU - take her up on the offer and get the custom design at a discount
YANBU - offer to pay more / choose a cheaper item