For what it's worth... I too get forbidden responses from the REST API and I too retry the Price request and then it usually works. In fact, I retry up to 10 times as sometimes it gives me forbidden repeatedly. It can happen getting Navigation Nodes as well as Prices. It isn't allowance and looks to me more like something server-side e.g. bandwidth throttling perhaps? I just retry failed requests in a loop and haven't figured out any proper fix but the retry loop works fine so not a big problem