The last time I was in Japan, I visited the large 108 yen store called Daiso in the trendy neighbourhood of Harajuku. It's located right on Takeshita Dori not far from Harajuku Station.
They sell Battery operated LED Decoration Lights for 108 yen. They come in 2 colours; White and Colorful Mix. They also come in 2 different modes, Flashing and non Flashing. I picked up the Flashing White LED package and the Non-Flashing Color LED package. The White version only come with 8 lights whereas the Colorful one comes with 10 lights.
After you remove the light string from the package, you have to install two AA batteries into the battery pack. On top of the battery pack, there is a black on/off switch. I switched on the light string and this is the result. There are 4 different colours in this string: Yellow, Red, Green and Blue.
There was sufficient lights for me to light the second and third floors of my modified Lepin Cafe Corner modular building. The wires are long enough that I can place the battery pack outside the building and feed the LED string into the second floor balcony door. I added some 1x2 plates with hooks to hold the light string in place on the second floor, then I ran the remainder of the LED string up the stairwell into the third floor and placed the remaining lights next to the windows.
Here I've opened the packaging for the White LED lights and installed the batteries.
Here's the modded Lepin Cafe Corner modular with LED lights on all 3 floors.
So, the two LED light strings cost me a total of 216 yen which is around $2 USD. For that low price I can light up 3 floors of my Lepin Cafe Corner modular building.