LLM Cost Calculator — C# source

// llm-cost-calculator — C# port: per-million-token cost math for LLM workloads.
using System;
using System.Collections.Generic;
using System.Linq;

namespace LlmCostCalculator;

/// <summary>Multiplier applied to Batch API pricing (the standard 50% discount).</summary>
public static class LlmCost
{
    public const double BatchDiscount = 0.5;

    /// <summary>Price pair for a model or a custom rate card. null = unpriced.</summary>
    public sealed record CostRates(double? InputPerM, double? OutputPerM);

    /// <summary>One workload to price.</summary>
    public sealed record CostInput(long InputTokens, long OutputTokens,
                                   long Requests = 1, bool Batch = false);

    /// <summary>Pricing projection of a model — only these three fields
    /// of the full model record matter to cost math.</summary>
    public sealed record Model(string Id, double? InputPerM, double? OutputPerM);

    /// <summary>Cost in USD, or null when either rate is unpriced:
    /// ((in/1e6)·in + (out/1e6)·out) × requests × batch discount.</summary>
    public static double? CostFor(CostRates rates, CostInput input)
    {
        if (rates.InputPerM is null || rates.OutputPerM is null) return null;
        double b = input.InputTokens / 1e6 * rates.InputPerM.Value
                 + input.OutputTokens / 1e6 * rates.OutputPerM.Value;
        return b * input.Requests * (input.Batch ? BatchDiscount : 1.0);
    }

    /// <summary>Output tokens per USD: 1e6 / OutputPerM, or null when unpriced.</summary>
    public static double? TokensPerDollar(Model m) =>
        m.OutputPerM is null ? null : 1e6 / m.OutputPerM.Value;

    /// <summary>Cost every model for one workload, sorted: cost asc,
    /// unpriced last, ties by id asc — the TS comparator verbatim.</summary>
    public static IEnumerable<(string Id, double? Cost, double? TokensPerUsd)>
        CompareModels(IEnumerable<Model> models, CostInput work) =>
        models.Select(m => (m.Id,
                            CostFor(new CostRates(m.InputPerM, m.OutputPerM), work),
                            TokensPerDollar(m)))
              .OrderBy(r => r.Item2.HasValue)          // priced rows first
              .ThenBy(r => r.Item2 ?? 0.0)             // cost ascending
              .ThenBy(r => r.Id, StringComparer.Ordinal);

    public static void Main()
    {
        var models = new[] {
            new Model("sonnet-class", 3.0, 15.0),
            new Model("budget-class", 0.5, 2.0),
            new Model("preview", null, null), // unpriced model
        };
        var work = new CostInput(InputTokens: 12_000, OutputTokens: 3_000,
                                 Requests: 250, Batch: true);

        foreach (var (id, cost, tpd) in CompareModels(models, work))
            Console.WriteLine(cost is null
                ? $"{id,-14} unpriced"
                : $"{id,-14} ${cost:F4}  ({tpd:F0} output tokens/USD)");
    }
}