I agree with skyrocketneko and Peppermint Truly, and here's why:
It looks, from the boxes, like the ponies and accessories might possibly have encountered mold at some point (you can see it on the frog float's tail and the blue ribbon right there in the photo, not to mention the state of that box). Mold alone can be a huge deterrent for any buyer--even if the surface is cleaned, it can still leech into the plastic and devalue a pony in ways that might not show for some time. The pink baby's mane looks discolored at the base of it right above her head, which could mean she's moldy inside (of course I would have to see other angles to be sure it's not a shadow). I see the plastic around her head, but it doesn't mean she was never exposed to water. Both floats show wear, and with the condition of the one still in the box, I wouldn't think its contents would be in great condition either--the box is so damaged, it's hard to even see the state of what's in there.
You've gotten some good advice from two very long-term collectors, PT in particular who is this community's expert on Mexican ponies. Not all foreign ponies are worth the same, either; a sea pony won't automatically be the same value as a flutter pony, even if they come from the same country.
Bottom line is, you can't judge the value of a pony based solely on its country of origin, or it's type. Salty and Chief are Big Brother ponies, but Chief is almost always going to go higher than Salty even if both are mint condition.
You can pick up a peach-colored Italy Peachy for $5, but a brown or white one will set you back close to, if not over, $100. Year 5 sea ponies (White Cap etc) are always going to be some of the rarest and most sought-after ponies, especially with the lighter colored ones so prone to discoloring (White Cap is famous for this). No country isn't always an indicator of some extreme value.. I've sold several NC family ponies for less than $15-20 each, in mint or near mint condition, and that was what people were willing to pay.
That blue Lickety-Split is an example of an exceptionally rare pony, it's no surprise she went for that high. She's one of few even known to exist at all.
I hope this explains the scale of value to you a bit, why it can differ greatly from pony to pony, and why you can't always expect top dollar. Ultimately the value of a pony is set by the collecting / buying market, and any educated collector will know when too much is too much.
Unfortunatly the watercolor seaponies as a whole are one of the least sought after sets, from forum comments I'm assuming it's due to the hair type that was used on these. Many people dislike the look and texture. The whitecap with box you linked to, she's not only harder to find, but the packaging she was in (even in that condition) and how sought after she is, is what made her price go that high. If I had the money handy I would have bought her for the box and put a nicer pony inside. 
