The chip - ATmega328P-PU - is the dearest item. About $5 if you buy one off, but you can get them for about a dollar each if you're prepared to buy 50 or 100. Actually, there are cheaper ATmegas than the 328 that would do the same job, but I had a few of 328s hanging around. The other parts are only pennies - even when you're buying in small quantities:
| ATmega328P-PU | $5.00 |
| Four resistors | $0.12 |
| Seven 3mm LEDs | $0.35 |
| Scrap of stripboard | $0.50 |
| Battery connector | $0.30 |
| Filament for printing case | $0.20 |
| Total | $6.47 |
If you wanted to knock them out in bulk, then with a simple single sided PCB and an injection moulded case, you could easily get the material costs down to less than $1.50 per item.
You need a a USBASP programmer to flash the firmware into the chip (or you can use an Arduino instead, if you have one). And a 3D printer to print the box!
Would you like a tester? I could make another one and mail it to you, if you like.