Call for Price:
http://www.opencart.com/index.php?route ... on_id=4067
Ask a Question:
http://www.opencart.com/index.php?route ... on_id=4098
The main difference is this:
- "Ask for Price" text is a link, which leads to the product's page, and automatically activates the "Ask a Question" tab, as well as it scrolls down the page to the "Ask a Question" tab, so the costumer should only fill the form, and ask for the price of that particular product (or anything else if he wants), and you'll receive the question at your email (see the demo from the above link for "Ask a Question" extension);
- Email icon instead of the Phone icon;
- "Ask for Price" text instead of the "Call for Price" text (but you can change this as you want);
See the pictures and the demo from those two extensions (links from above) and you'll get the idea

Installation is as simple as it can be, just open this, find that, after that add this, and it's done. No files are overwritten, no complicated database queries, no trouble at all.
If you need any help, free installation is on its way. Just contact me by leaving a comment, or via forum's PM (GoGo OpenCart).
OpenCart versions supported: 1.5.1.x (As soon as I test it with other versions of OpenCart, I will update the compatibility list, so stay tuned

Extension's page: http://www.opencart.com/index.php?route ... on_id=4139