For the most part I agree with the parent, though I will be more blunt (though not necessarily short-winded).
You took it upon yourself to develop *something* which makes your (and your team's) job easier. However the only way I see you getting paid for this developed application is so effective that it actually renders you redundant, if it does (and I were in charge) I would not pay more than .5x your salary for the "application" however realistically probably not more than 1K or so, since realistically you can't provide a proper support structure so they would be buying it as/is. If it renders you and your entire team redundant then maybe it would be worth closer to 1x your salary (again lack of a proper support structure really kills the deal at this price point)...
Now bottom line... You did this for a reason, you got something out of it, be it knowledge or what-have-you. Also keep in mind that unless you have a contract which is VERY clear on who owns what property your state might already have laws which sell out your work-product regardless of whose time it was produced on. Even if your state isn't one where they can claim your after-hours work product, if they wanted to make a play for it then you would have to hire a lawyer to prove that (1) you created it without company resources (2) you created it on personal time (3) you had a personal reason for creating it. The third one might be hard to make the case for since it makes your PROFESSIONAL life much easier which would be a professional reason. Either way you are already at a disadvantage. They already have a lawyer (although he/she might not be the top of their class), you will have to find one who thinks you have either (1) a case and a little bit of money (2) no case and a ton of money.
So basically as for actual monetary compensation... It is out of the question... Won't happen. That said these are also the kinds of things which show your value to the business and frankly can be used to justify a promotion, assuming one is due and you weren't being a total schlub at work while developing this application in your "off-time".
Now in the future... If you are developing an application for work in your personal time. Please have a friend slap you. If the application is for work then either know you are working for free or clock the hours worked, this way they can object immediately if they don't want you working on the application in question.
Finally... When did "Senior Systems Administration" stop including at least a smidgen of development?