It will be very expensive if you're looking at something permanent that turns your guitar into an electro-acoustic. Several hundred dollars or more. I once considered doing this for a full-sized guitar but I got scared off when told how much. I ended up buying an electro-acoustic for $300+ instead.
You can also consider a soundhole pickup which you can install or remove yourself. Just that the wires can be quite annoying. For the ones I've used (Seymour Duncan Woody and another one I bought from Maestro), a wire extends from the pickup itself and ends with an output jack at the other end. I had to find a way to keep the wire hidden away and tied the output jack to the end-pin so my strap would still fit. Some can also be installed permanently, i.e. the wires are inside the guitar and you only need to install the output jack, but you will need a guitar tech for this.
They cost anywhere from below a hundred to a few hundred dollars depending on the quality. I've used the temporary ones for my performances and they were OK. Used an affordable guitar so I wasn't too bothered about the sticky tape keeping the wires neatly hidden away on the back and sides of the guitar, and as I said earlier, I tied the output jack to the end-pin so my strap would fit.
