* Consider using `Text` instead of `String`, as `String` is a linked-list of `Char` and quite inefficient

* Consider refining some of your fields into more constrained types. Sort orders probably only have a couple of valid values, and might reduce to an enumeration instead of `String`.

* In `instance ToQueryString PriceRequest`, I'd be surprised if those type annotations are necessary. The inferencer should be able to work out that `req` is a `PriceRequest`.

* In `instance FromJSON AggregatedSnapshot`, I think the `read`s should go. I think you want to use the `Float` instance of `FromJSON` instead. (What the `read <$> ...` is doing is parsing to a string, and then using the partial (!) `read` function on the result. JSON responses that have a value that fails to read for that key will crash the program.)

* In `instance FromJSON AggregatedSnapshot`, consider `withObject` here too, which encapsulates the pattern you're using.