Just to get this said first and hopefully avoid a war, or somehing. What follows is not said out of malice because you are trying to 'steal' my idea (which wasn't mine to begin with), or because I want EUCLiD to be the only level editor around. I don't really care what happens to it, it was a practice project to help me learn visual basic. I am still working on it, but if it dies, I don't care. No, I say this to prevent you wasting your time an money and effort.
As plans go, this is bad. Not so much the application itself, but the idea of having to pay to be able to have more than one map in development.
Think for a minute: Most mappers around here are good enough to code the whole level themselves, even add whole new facets to the game (infected AI, gravity). They don't neeed a tool to do it for them. Less-good mappers can learn quickly from the threads here and studying others' maps. Now, you suggest making something to ease the work. Fine, do so.
Just don't ask for money.
Yes, I know that you can use it fine without paying, but you are limited to one map at a time. Notepad (or equivalent) is 100% free (ignoring the cost of the computer, but how can you make levels without a computer), and you can make as many levels as you hard drive allows at once. Given a choice between Notepad and your app, I know I'd pick Notepad. It's free, works instantly and freely with patches, and can use every part of the code.
I guess what I'm trying to say is this: By all means make a level creator/editor. Just don't expect people to pay for things if they can legally get them for free. Which they can.