It takes a lot of email messages to make up a single telephone conversation. I don't have a mathematical formula, but my guess is that if the conversation is of some small importance, 11 email messages may make up a single, short telephone conversation. If the conversation is seriously important, the number of emails rises proportionally. Why? It's the wrong medium for the exchange.
Oh, you'd rather email someone than bother them with a phone call? It's bothering someone to have to respond to five emails about the same subject.
If it takes a lot of email messages to make up a single telephone call, what's the math on a face-to-face conversation? If I had to guess, it might be something close to 63 email messages to make up single face-to-face conversation. And the communication, even though it's in writing, is poorer.
The need to have whole conversations over "mail" ended with the invention of the telephone. Email is very much like postal mail. You wouldn't want to have an important conversation over mail, would you? It would take too long, and it would lose something, wouldn't it? Email is even more like a telegraph.
How to Decide to Use Email
Email is excellent when you need to send information to another party. It's also excellent when you need an answer that doesn't require any real conversation. Email is a poor medium when the topic is important or when a discussion is necessary (like your proposal and pricing). It's a horrible medium for resolving issues, problems, or challenges. It's also a horrible medium for many of the sales conversations that are attempted through email. Are you sharing information? Need a quick, easy answer? Nothing easily lost in translation? Send an email. Important issue or request? Requires dialogue? Much of what needs to be communicated can't (or won't) be communicated effectively over email? Schedule a face-to-face meeting, a video chat, or a phone conversation (in that order).