Personally, the many thousands of dollars I would spend on development of a control panel would far outweight the cost of purchasing leases. One of the biggest things we deal with is being able to move a client from one control panel system to another, and have it still be familiar to them. The major two players (cpanel & plesk) are positioned in such a way that the majority of transfer IN accounts are going to be from these places. When a user already has to make the hard choice of changing hosts, the last thing they want to do is learn a new control panel system too.
I've always found that most of the custom control panels that varoius companies use are pretty inadaquate. Yes it provides the client with access to email accounts or uploading files, but the basics of MX Record changes, DNS changes, Adding additional domains, editing databases or even making a simple backup are just so cumbersome in a custom system that the client literally falls in love with you as a company when they change to something much more user friendly.
Having a unique control panel isn't really a positive selling point. I recommend sticking with control panels that are popular and established, ones that your potential clients are probably already familiar with.
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Its better to note the number of customers as you will have to provide support for the control panel, which can be avoided if its traditional ones. Also the cost to develop will be high and right team to develop will not be easy to find (Cpanel developed in 1999 and has been updating since then).
If the clients are of a pretty good number then surely it will minimize the cost and give more control with time.