Building a custom Large Language Model sounds like something only AI research labs can afford. But in 2026, businesses have more options than simply training a massive language model from zero. A company can customize an existing open-source model, fine-tune a foundation model, build a private LLM with Retrieval-Augmented Generation (RAG), or train a completely new model from scratch. The cost can therefore range from thousands of dollars for a focused model customization project to millions of dollars for large-scale pretraining . The biggest mistake businesses make is asking: "How much does it cost to train an LLM?" before deciding what they actually need. If your goal is to make an existing model understand your company's terminology, follow a particular response style, or perform a specialized task, fine-tuning may be enough . If you want to create a new foundation model with your own architecture, tokenizer, massive training corpus, and model weights, the investment ...