I think the nearest antonym, in that context, is fully:
It costs fully $200.
But fully is relatively rare, especially in speech; if you went around saying fully <amount> anywhere near as often as people say only <amount>, it would be a noticeable quirk. Also, fully is a bit more restrictive; for example, whereas you can say "only about $200" if it's actually $205, you can't say *"fully about $200" if it's actually only $195. (But in that case you can say "almost $200", which conveys the same idea — almost implies that it's high — and is much more common anyway. And, incidentally, if the price is over $200, then the usual expression is "over $200", which, again, conveys that it's high. It's only if it's exactly $200 that we need something like fully to express our reaction.)
There are a few other similar expressions, such as "a whopping $200" or "a full $200", but none of them are all that common. Instead, I think it's more common to convey your feelings on a price with tone of voice and general-purpose "wow" expressions:
It costs $200!
It costs $200??!!!
It costs 200 <expletive> dollars!
optionally augmented with other aspects of the wording:
It costs more than my smartphone!
It costs $200. Not worth it.
That little piece of plastic costs $200.
It costs $200 . . . let's shop around.
It costs $200, because they know they have a monopoly.
It costs $200, 'cause, not like I really needed to make rent this month, anyway . . .
(Not that tone of voice is always successful. I can recall quite a few times where one person expressed surprise about a number, and another person had to ask, "Were you expecting higher, or lower?". Of course, one nice thing about conversation is that we can clarify these things interactively, but it does kinda ruin the moment. :-P )