Assuming 10W standby consumption, for an overnight period of 14 hours (6PM - 8AM) that equates to 0.14KWh, which is 51kWh over the year. In the US, this would cost just over $5 (at 10c per kWh), and in the UK just over £5 (at 10p per kWh). You could double this for thirsty peripherals, and halve this for frugal peripherals.
For peripherals in this sort of range of power usage, it wouldn't be economical to use one 7-day timer per device - especially when you factor in the power consumption of the timer itself! One timer supplying a bunch of devices might be beneficial, however.
The best thing is to institute a process of manually switching off certain devices at the end of the day. But being the last one to leave the office at night could really be a chore!