I think the only really expensive ones are the ones with the Y3 adult sea ponies.
In terms of the baby sea ponies, you probably would get more if you had stickers for Water Lily, Ripple, Sunshower, Beachcomber because they didn't have puffy stickers outside of the UK. Any of the first set are fairly common though I think. Although if they're in good condition they might get more attention (on original backing and such).
So I think Pumpkin_Tart's suggestion of around $10ish makes sense if in good shape.
I'll let the experts weigh in as well, but I think aside a couple of exceptions (Y3 adult sea ponies and maybe the 2nd set Rainbows to a degree), the puffy stickers available in the US are not super HTF or expensive.
Most of the expensive ones tend to come from the UK or from Europe, including Nirvana in European releases, particularly Italian ponies.