### Check for existing issues

- [x] I searched open and closed issues before filing - [x] #30738 concerns string coercion inside DashScope tier data and #21249 concerns missing flat prices during budget enforcement, so neither covers this pricing-model mismatch

### What happened?

The DashScope cost calculator applies graduated, income-tax-style slicing to `tiered_pricing`, while Alibaba Model Studio selects one tier from the total input-token count and bills all input and output tokens at that tier's unit prices

The proxy budget reservation code already implements the request-size model through `select_tier_for_input`, so reservation estimates and post-response spend accounting also disagree with each other

This can materially understate logged spend and weaken budget enforcement for large Qwen requests. Cached input is especially inconsistent because the current calculator tiers the cached-token count independently from zero instead of selecting a rate from the request's total input size

Alibaba's current pricing rule is documented here:

https://www.alibabacloud.com/help/en/model-studio/model-pricing

The relevant rule says the unit price is selected by total input tokens i…