Hi.
I think charging some monthly amount it's the equivalent of Plesk/Cpanel licenses (personally, I want to make a switch from Plesk).
Solutions:
1. Google Ad's, non-intrusive, could generate some money.
2. Advertise on www.buysellads.com.
3. Something I did see on SysCP Mantis: Issue sponsoring (some people would pay to see a bug/feature resolved/added sooner).
4. Site Donations, with optional links in main site
5. Offer services (maybe with external help/people): Install, Config, Setup, Customization (standard or customized themes)
Note: adding an API sooner will lead Froxlor to commercially environment (HostBill added in v1.4.6 support for ISPConfig 3, even it's in beta stage and has an incomplete API: just create and delete account, without account suspending. WHMCS will not support any control panel without a proper API).
My 2 cents.
Regards,
/Sorin