ASE 11.0.3.3 is completely free - both for development and deployment. This means that you can use it to host your clients, and your clients expecting MS-SQL will hardly see the difference.

ASE 11.9.2 is free for development, but is somewhat obsolete.

ASE 12.5.0.1 is available free for development (there is a specific Developer's Edition). It's got lot's of improvements on the previous versions. You can't use it (legally) to run a production server. ASE (Adaptive Server Enterprise) runs for something like $800 for a linux server, but for an internet license you should expect to pay something like $10,000 (steep, I know, but try Oracle...).

As someone mentioned, the alternative from Sybase is Adaptive Server Anywhere (ASA). It's a different code-line, developed from the Watcom SQL Anywhere system. It's light-weight, but supports stored procs, triggers, etc. and should be quite a bit cheaper than ASE.

Michael