Is it possible to add a retina display on the iPhone 3GS?

I can't afford upgrade just yet.
I was wondering if the retina display could be added to the iPhone 3GS that I currently own?

No. It lacks the hardware and firmware needed to drive a retina display.

In any case, many other phones have higher resolution screens than Retina at a far lower cost than Apple charge for it on their products.

No you can't. Retina display is a hardware feature, as in a physical thing on or in the phone (i.e. The screen), as opposed to software which is everything to do with the operating system and stuff, things you can't physically touch. You can't just add retina display to a 3gs because that would require the screen to be replaced, which you can't do because that would include a load of complications.

And btw you won't find a phone with a screen sharper than retina display for less money than a 4s. There are phones that come as close as makes no difference (my phone has 320ppi compared to 326ppi on the 4s, absolutely can't see the difference.), but no phones that have sharper screens for lower prices.