Minimum Spot Bid Price and "hardware costs"

Question

While trying to update the max price per hour for a spot VM in the portal, I am unable to set it lower than 40% of the pay as you go rate. This is despite all available pricing information sources stating that the current spot price is much lower (~20% of PAYG).

When trying to set it to a lower value, the portal displays the following error message:
"Enter a price greater than or equal to the hardware costs (US$0.23160)"

The spot pricing for the NCas_T4_v3 according to this page is $0.1227/hour at the time of writing this question.
A query to the "Retail Prices REST API"(available here) specifies the current spot price as $0.122667/hour.

The pricing history for the VM type also shows a drop in price:

Running the VM at a max_price of -1 results in a charge of ~$0.24/hour. I managed to set a lower bid using the Azure CLI interfaceas follows:

az vm update --resource-group MyResGroup --name MyVmName --max-price 0.13

The VM did run for about 20 min after which it was evicted. That seems like expected behavior.

1. What is this minimum "hardware cost" and how is it related to the spot prices mentioned elsewhere?
2. Is it possible to get fine grained spot pricing history? Something like min and max within a particular time window.
3. Is the inability to set max price in the portal a bug? Considering the CLI accepted the max-price without issues.

Answer 1

The "hardware cost" represents Azure internal cost baseline for operating the underlying infrastructure, it is essentially the minimum price Azure considers viable to run the VM and varies depending on the region, VM type, and current capacity constraints.

Spot prices fluctuate based on available capacity and demand. While the spot price might be lower than the hardware cost at a given time, Azure likely enforces the hardware cost as the floor price to avoid financial losses in certain situations, such as sudden demand spikes or capacity shortages.

Azure provides spot pricing history on the Azure Pricing Calculator and through the Retail Prices REST API. However, this data typically reflects average or recent prices, not detailed min/max prices over time.

Fine-grained historical data with specific min and max values over time windows is not readily available through Azure public interfaces. For more precise data, you might need to set up custom monitoring of spot prices by querying the API at regular intervals.