But let's take a use examples.
In the case of someone like Fred Wilson, he will return your email if it is something that he is interested in [2] and not return it if he isn't interested. Even if you are on his "A or B" list. (And even if you are on his A list he won't necessarily take the time to read or understand your entire email.[1])
So your idea seems to be "Fred will know that I (a nobody) have paid .50c or $5 and therefore be more likely to read my email. After all you are not pitching this idea as a replacement, merely an additional way to get someone's attention. The price has to be below other methods (a billboard on the 101) or express mail.
So this is not going to solve the email problem of an important person. It's only going to put certain email, and this is important, hopefully with a better chance of getting read.
The problem is it really doesn't solve any problem for Fred. He will still get a shitload of email. All he will see is that 15 or 20 messages a day people decided to pay to have him see their message. So this is really a benefit to the sender not to the recipient (like Fred anyway).
[1] I've gotten responses in 5 minutes and in 5 hours and I've gotten no response. Frequent occurrence. Generally if you don't get a response w/i 24 hours you aren't going to get a response because it's not something he wants to deal with.
[2] In order to even begin to approach doing an idea like this you would have to see the mail flow of a power user like Fred. My guess is that he gets so many emails of so many purposes (companies he's funded asking questions, people he has met, pitches, Michael Bloomberg, other VC's) that it would simply not be practical to begin to give out an email address or post an address where people have to pay to email him.