ok, basic 555 circuit as shown below.
couple of capacitors, couple of resistors, and one IC. Max cost about £1. Transistor to power relay - depending on the type of relay, probably no more than £1.
Total cost, therfore - about £2.
1 Second cycles should be fine - do they need to be extremely accurate?
components for 1 second cycles - I assume that means that it flashes once a second, not on for a second,and then off for a second?
Anyway, for a frequency of 1Hz, use this formula:
Frequency = 1.44/((R1+R2+R2)*C)
(i will work out the values when i've woken up)
Also, if you are not using a solid state relay, you will probably hear the 'clicking' sound of the relay - which presumably you don't want.
Tim