If the cost of using Power Apps is a concern, there are alternative solutions that you can explore to meet your requirements for client login, file download permissions, and case creation in your CRM.
One possible solution is to use custom development with a web framework such as ASP.NET or Node.js to build a web application that integrates with your CRM and provides the required functionality. This approach would require more development effort than using an out-of-the-box solution like Power Apps, but it would give you greater flexibility and control over the user experience, functionality, and cost.
Another option is to use a third-party app or tool that specializes in CRM integration and client portal functionality. There are several options available in the market, such as HubSpot, Salesforce Community Cloud, and Zendesk. These platforms provide pre-built components and templates that you can use to create a portal that meets your requirements for user authentication, file access permissions, and case creation. However, these solutions may require additional licensing and subscription fees, so it's important to evaluate the cost and features carefully.
Finally, if your CRM system has a public API, you can develop a custom web or mobile application that interacts with the CRM system directly, without the need for a portal or middleware layer. This approach would require more development effort and expertise, but it would give you the greatest degree of flexibility and control over the user experience and functionality.