There is the student version of C18 that many people overlook
From the C18 license
- LICENSE GRANT. Subject to all of the terms and conditions of this Agreement, Company grants You a non-exclusive, non-sublicensable, non-transferable license to install Software on a single computer and use the Software with Company products. Notwithstanding the foregoing, if You downloaded the "Student Edition" of the Software from the web, You may install and use such version of the Software on an unlimited number of computers for commercial or educational use.
So if you can do without the optimisation and extended instruction set after 60 days, you can use the student edition for commerical or educational use.
Alex