The heart turned out about 6.5x6.5 cm.
What you need:
A computer with CAD and CNC mill (Depending on the shape that you want to make and the material you choose to work with. If you are going to do some really simple shape you can manage without this)
1 Battery (C2032 3V)
1 Red 6mm LED
1 Tilt switch
2 Resistors (16 ohm)
2 M3 Bolts with nuts
2 pieces of 100x100x6 mm plexiglass
(optional) some heat-shrinking tube
This probably cost me around 20$-30$ but i bought enough parts to build at least 30 of them so building one will probably cost you around 5$-10$ since single parts are usually more expensive than packs of them. The two most expensive parts are the switch and the battery.
It does not take much experience but basic soldering skill and basic CAD knowledge will make it a lot easier.
I used http://led.linear1.org/led.wiz to find out which resistor to use.
Step 1: Shaping Your Figure
Step 2: Shaping Your Figure 2
If you are confused by what i just said please have a look at step 4 where both halves are printed.
Step 3: Printing the Shape
Note again that this is the lid and how it doesn't have any space for the Battery, LED and resistors.
Step 4: Printing Your Shape 2
Step 5: Soldering the Circuit
The tilt switch i got was a non-mercury one which from what i can tell is composed of a metall ball rolling inside a tube to turn on/off. It does flicker a bit if not completely still. If you have access to a mercury tilt switch i would probably recommend using that insead (it is hard to come across those since electronics may no longer contain mercury).
The lower picture is of the first time i assembled the heart, it still needs some sanding to say the least.
Step 6: Sanding
All of the tabs have to be removed as eel as the entire outside of the heart. BE CAREFUL not to scratch the front and back of the heart since plexiglass is really easy to scratch and it will look really ugly. I recommend using a smooth sandpaper just to be extra careful.
Also take cars of the plexi, note that there is a piece of corrugated fiberboard protecting the heart from scratches from the vice.
Once i was done sanding the edges and removing the tabs i washed the pieces in water to remove some of the dust from the sanding.