If you truly feel inspired by someone, don't donate. Support them.
There are multiple ways of supporting people - buy their signatures, click on their sigs and buy the products/services or heck, just press the thanks button!
You might not realize but if enough people do this, it goes a long way.
And every user is different, setting donations criteria could encourage others to become Execs for ulterior motives which the forum would never want.
Execs share their tips/hacks because they feel happy by sharing, not to make a quick few bucks.