If you are just trying to get some already clean data into the right format and structure for Tableau with a few minor calculations, then Tableau Prep is fine and certainly less expensive. For most cases Alteryx is just too much of an amazing tool to bypass. Alteryx is so much more capable as far as data analysis, transformation, building user friendly interface controls, complex algorithms, flexible custom report design (such as pdf reports with accounting presentations and narratives), more input / export format and connectors, iterative macros, better community support, better product support, etc.
It is possible that you could close some of the gap via custom Python and R programming in Tableau Prep, but it would still fall short in macro / custom reports. You would also burn a ton of work hours, create updating hassles and have a much less user friendly and slower tool overall.
When I have run into cost issues with customers in the past on Financial Management System automation projects, I typically recommend that the companies hold off on Tableau Server rather than Alteryx. Alteryx on its own can still unify all of your sources / outputs, automate processes, generate custom reports, create and upload custom interactive dashboards, automate emails, upload reports to a CRM, etc. The main thing that you loose is the ability to do Tableau's powerful parameter controlled calculations on the fly and writeback. Tableau Server typically is still implemented in phase 2 of the plan after we boost profitability.
You can always try a Tableau Prep demo to see if you can get by with its more limited capabilities.
