Clientexec is a billing solution for web hosting providers. The price for a single license of Clientexec varies by the type of license. As of November of 2015 the price for a leased license that is self hosted is $12 per month when billed annually or $16 when paid month to month. The owned self hosted license (no link back to Clientexec site) is $499 and includes 1 year of support. With a $50 fee for each additional year of support.
If you want Clientexec to host the installation for you. The price is for remote basic is $15 per month when billed annually or $20 for month to month. Remote basic is limited to 3 staff users. Remote pro allows 6 staff users. Remote pro costs $45 per month when billed annually or $55 for month to month.