MSP430 waking up every millisecond, what power usage to expect?
The instruction time is 1/25M = 40ns.  Assuming that your interrupt routine is around 20 instructions (it's probably less from the sounds of it), then the routine will last 0.8us.

Assume that the active mode current is drawn for 6us after the ISR has finished, at which point it instantly changes to 2.1uA (for simplification).

In 1ms, the routine will be firing for 6.8us (in active mode) and LPM3 will be on for 993.2us.

If the values you quoted are accurate, then assuming your micro has a 3V supply, 99.32% of the time it will consume 3 x 2.1u = 6.3uW.

For the rest of the time (0.68%), it will consume around 25 x 195u x 3 = 14.6mW.

Since you have the power consumption and the relative times, you can work out the average power.