Time is money in the auto repair business. It's much faster and easier for a mechanic to replace the entire driveshaft assembly with both joints on it than to mess around replacing a CV joint on your old driveshaft. Removing the old CV joint from the shaft, disassembling and inspecting the other CV joint on the shaft to make sure it is still good, reassembling and repacking both joints with grease and installing the boots and clamps is a messy and time-consuming job. So that's why your mechanic is trying to give you the "shaft." He isn't trying to cheat you. He's only trying to save himself some time and effort.
The cost of replacement shafts for most FWD cars today has dropped to the point where a complete shaft assembly with new or remanufactured CV joints costs little more (or in some cases no more!) than a brand new replacement joint. That's why most mechanics have gone to swapping shafts instead of replacing individual CV joints.
When the shaft is changed, your old shaft and joints are exchanged for the replacement shaft. Your old shaft is then returned to a company that specializes in shaft rebuilding. Your old shaft is then rebuilt using new or remanufactured joints. The shaft then goes back into the parts distribution pipeline and is sold to the next person who needs one. That's how the system works. It's recycling in action, and it actually saves consumers a lot of money.
If you're pinching pennies and/or don't plan to keep your car for a long time, you can save some money by asking for a shaft with remanufactured, rather than new, joints. The warranty won't be as good, and the joints may not last as long as brand new ones, but you get what you pay for.
Shafts for import vehicles typically cost about 30% more than those for domestic vehicles because there are more different designs of import shafts and joints (some of which can be very difficult and expensive to obtain).