Is it possible to create a custom (drop-down-list) field in Request properties, that would invoke additional (numeric) field accoding value in first field?

Example:

We can create a field called "Price", which is drop-down list:

[ < 1000 € ]

[ > 1000 € ]

If we select [ < 1000 €] than nothing happens, but if we select [ > 1000 €] system would enable additional numeric field (perhaps "Exact price"), where you should write numbers.

I know it's not possible by default, but perhaps with the help of "Menu Invocation" or "Custom Triggers" (because I don't understand these) and some coding/scripting might be possible?