you can find out exactly how much it will cost you by first checking your electric bill and finding out how much you are charged per kilowatt hour (kwh). Everyone's charge will be different, depending on your location, electric company and your usage. Many higher usage customers are penalized with charges in tiers. What this means is that the more kwh you use per month, the higher the rate charged. They claim it's supposed to be a deterrent for high usage, what it actually is is price gouging lol.
Anyways, for the sake of discussion, let's say your rate is $.10/kwh. a 1000W appliance running for 1 hour = 1 kwh. So, if your bulb is 250W, it uses 1 kwh in 4 hours (250 x 4 = 1000).
based on 24 hour lighting, and every 4 hours is 1 kwh, the energy used will be 6 kwh/day. So, at $.10/kwh, your daily energy usage for that bulb will be $.60.
In a 30 day month, your usage will be $18.00
That is how you figure it out, now you just need to check your bill to see what your rate is. And make sure you read the charge rate carefully, sometimes there is a rate of charge and also a usage fee or something like that lol. There are high usage customers that pay about $.45/kwh for their higher tiered usage here in CA. Base rate is around $.11 or so