# Price and wait

By default, the exchange does not pick the offer with the lowest charge. It picks the offer with the
lowest **cost**: the estimated charge plus the **wait cost**. The wait cost is the money value that
you give to the time that you wait for the answer.

This applies to the `price` strategy, which is the default. The `cheapest` axis uses it too.

## How the cost is made

The exchange adds two amounts for each offer that meets your limits:

```text
cost = estimated charge + (predicted wait in ms × wait value)
```

- The **estimated charge** is the price that the exchange expects for your request.
- The **predicted wait** is the time from the start of the job to the end of the answer. It is
  based on measurements of the provider. See
  [How the exchange measures performance](https://inference.ai.exchange/docs/performance-measurement.md).
- The **wait value** is the price of one millisecond of wait, in nano-USD (one billionth of a
  dollar). The default is 200. That is $0.0002 for each second, or $0.72 for each hour.

The offer with the lowest cost wins. The exchange then runs the normal steps: the price cap, the
hold, and the charge for the tokens that the provider really used.

## Example

Two offers answer the same request. The wait value is the default, 200 nano-USD for each
millisecond.

| Offer | Estimated charge | Predicted wait | Wait cost | Cost    |
| ----- | ---------------- | -------------- | --------- | ------- |
| A     | $0.0100          | 4.0 s          | $0.0008   | $0.0108 |
| B     | $0.0105          | 0.5 s          | $0.0001   | $0.0106 |

Offer B wins, although its charge is higher. You pay the real charge of offer B, which is never
more than its price cap. If you set the wait value to zero, offer A wins.

## Change the wait value

Set `exchange.rank.latency_nanos_per_ms` in a request or in a [preset](https://inference.ai.exchange/docs/rules.md). Use a whole number
that is zero or more.

```json
{ "exchange": { "rank": { "latency_nanos_per_ms": 0 } } }
```

| Value            | Result                                                                            |
| ---------------- | --------------------------------------------------------------------------------- |
| Not set          | The default of the exchange, 200.                                                 |
| `0`              | The exchange ranks on the estimated charge alone. An explicit `0` beats a preset. |
| A larger number  | A faster offer wins more often, even when its charge is higher.                   |
| Above 1000000000 | The exchange lowers the value to 1000000000.                                      |

The exchange refuses a value that is not a whole number or that is below zero.

## What the wait value changes

The wait value changes only the **order** of the offers. These rules do not change:

- An offer must still pass every limit, including your price cap.
- The price cap, the hold and the settlement work as before.
- The exchange never shortens your answer or sets your output limit.
- The `price_band_bps` setting applies to the cost, which includes the wait cost.

## When the exchange ranks on price alone

The exchange ranks on the estimated charge alone in these cases:

- The strategy is `fastest`, `smartest` or `auto`. These strategies have their own rules and ignore
  the wait value.
- The request is not a chat request.
- The wait value is zero.
- One eligible offer has no complete measurement. The exchange then ranks the whole auction on
  price, so that an offer with no data does not get an advantage.

To see why an offer won, read [Explain the choice](https://inference.ai.exchange/docs/rules.md).

## Next steps

- [How the exchange measures performance](https://inference.ai.exchange/docs/performance-measurement.md)
- [Purchasing rules](https://inference.ai.exchange/docs/rules.md)
- [Axis routing](https://inference.ai.exchange/docs/axis-routing.md)