feat(models): add cost estimation for OpenAI models

- Introduce `Cost` type and constants for token pricing.
- Implement `EstimateCost` function to calculate usage cost.
- Add `CostPerToken` struct and `OpenAIModelCosts` map for model pricing.

[Generated by Kommit]

Author: cowboy-bebug

internal/models/openai.go

@@ -12,12 +12,62 @@ const (
 	OpenAIModelO3Mini    openai.ChatModel = openai.ChatModelO3Mini
 )
 
+func IsSupportedModel(model openai.ChatModel) bool {
+	return slices.Contains(OpenAISupportedModels, model)
+}
+
+type Cost float64
+
+// https://openai.com/api/pricing/
+const (
+	// GPT-4o Mini
+	OpenAIModelGPT4oMiniInputCostPerToken       Cost = 2.50 * 1e-6
+	OpenAIModelGPT4oMiniCachedInputCostPerToken Cost = 1.25 * 1e-6
+	OpenAIModelGPT4oMiniOutputCostPerToken      Cost = 10.00 * 1e-6
+	// GPT-4o
+	OpenAIModelGPT4oInputCostPerToken       Cost = 0.150 * 1e-6
+	OpenAIModelGPT4oCachedInputCostPerToken Cost = 0.075 * 1e-6
+	OpenAIModelGPT4oOutputCostPerToken      Cost = 0.670 * 1e-6
+	// O3 Mini
+	OpenAIModelO3MiniInputCostPerToken       Cost = 1.10 * 1e-6
+	OpenAIModelO3MiniCachedInputCostPerToken Cost = 0.55 * 1e-6
+	OpenAIModelO3MiniOutputCostPerToken      Cost = 4.40 * 1e-6
+)
+
+type CostPerToken struct {
+	Input       Cost
+	CachedInput Cost
+	Output      Cost
+}
+
+var OpenAIModelCosts = map[openai.ChatModel]CostPerToken{
+	OpenAIModelGPT4oMini: {
+		Input:       OpenAIModelGPT4oMiniInputCostPerToken,
+		CachedInput: OpenAIModelGPT4oMiniCachedInputCostPerToken,
+		Output:      OpenAIModelGPT4oMiniOutputCostPerToken,
+	},
+	OpenAIModelGPT4o: {
+		Input:       OpenAIModelGPT4oInputCostPerToken,
+		CachedInput: OpenAIModelGPT4oCachedInputCostPerToken,
+		Output:      OpenAIModelGPT4oOutputCostPerToken,
+	},
+	OpenAIModelO3Mini: {
+		Input:       OpenAIModelO3MiniInputCostPerToken,
+		CachedInput: OpenAIModelO3MiniCachedInputCostPerToken,
+		Output:      OpenAIModelO3MiniOutputCostPerToken,
+	},
+}
+
 var OpenAISupportedModels = []openai.ChatModel{
 	OpenAIModelGPT4oMini,
 	OpenAIModelGPT4o,
 	OpenAIModelO3Mini,
 }
 
-func IsSupportedModel(model openai.ChatModel) bool {
-	return slices.Contains(OpenAISupportedModels, model)
+func EstimateCost(model openai.ChatModel, usage openai.CompletionUsage) Cost {
+	cost := OpenAIModelCosts[model]
+	estimatedCost := float64(cost.Input)*float64(usage.PromptTokens) +
+		float64(cost.CachedInput)*float64(usage.PromptTokensDetails.CachedTokens) +
+		float64(cost.Output)*float64(usage.CompletionTokens)
+	return Cost(estimatedCost)
 }