In order to close this question out, and in case anybody else is wondering, I will answer my own question. This is what I've found out.
Because of the nature of the relationship between client & server, there is really no way to get the value of the text input, store it in a ruby variable, and check it against another ruby variable. Ruby script only runs once and then is rendered, so while ajax may be able to continually run in the background and gather inputs, etc, the integration with ruby falls short when talking client-side only interaction. (Correct me if I'm wrong- after all, I posted the question to get everybody's input!)
The fix: I created a switch using hidden tags. When the form loads, the hidden tag is blank. After submitting the form, the update action checks the params of the newly entered data against the numbers from last month. If the conditions check out, it saves. If not, it re-loads the page with a message. If the message is confirmed, an ajax command changes the hidden tag to "true" which bypasses the comparison once it hits the update action again. Otherwise, the data is not saved. Problem solved!
I'm making this a community wiki answer in case anybody would like to add their two cents.