This kind of thing comes up frequently. You can't use Express or a web server to act as a 'user' so as to only buy one CAL. The Standard edition server is processing data (from Express) to 25 users, hence you need 25 CALs for it.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability