There are a couple of ways to deal with licensing your engine as far as not giving away the WHOLE thing all at once. Some folks (Power Render, for instance) will license the "object code and tools" at one price (affordable) and license the source code at another (EXPENSIVE!).
Personally, once I finish my engine and tools I have no intention of providing the source code! Sure, it may lose a licensee or two, but it''s less of a support nightmare to just have one or two versions out in the field rather than each place having some hack of your original you now have to try to deal with.
If you provide DLLs or libs, docs, and tools you should be in good shape and not end up revealing everything you''ve done for the past years... and you don''t have to worry about someone making a mess of your engine and advertising it as "made with the foo-power engine" (and the bad publicity such a thing can bring).
Not like someone died and make me Carmack, mind you
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Hope this helps some!