There are different tiers of auditors, each with its own reputation, experience, and price segment. You should start choosing by defining the price segment that suits you. Rates for different auditors can vary between $900-$2000, $3000-$10000, and even more. Then you can study and evaluate the site of the auditor's company, and how well it is designed. Be sure to visit the GitHub of the auditor you like. There you will be able to view all past reports of the projects they have audited, and determine whether they are well-known or notorious. You can study how often the audit company is mentioned in the media and in what vein. Some auditors specialize in a specific blockchain, for instance, Ethereum or TON. There are also universal auditors - you should choose an auditor with extensive development experience on the blockchain you use.