http://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLAllowDownloadFee says:
Yes. You can charge any fee you wish for distributing a copy of the program. If you distribute binaries by download, you must provide “equivalent access” to download the source—therefore, the fee to download source may not be greater than the fee to download the binary.
Which I take to mean that the source code for the TRIAL version must be made available for the same cost or less (free).
Furthermore, don't forget: http://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLRequireAvailabilityToPublic This means that someone else can take the source after paying one million dollars, compile it and distribute it. I believe this is how CentOS works.
By using RedHat as an example, the key would be first-to-market backed by support. People could re-share your source but they would have to bother. If your product is good and you continue to keep it fresh, then the risks of clones is reduced.