Since I don't have a separate meter for my tub, I don't have precise figures. When I bought my tub (Hot Spring Jetsetter LX, 210 gallon) last winter, I was told to expect about $50/month to keep it going. The reality was that I barely noticed any difference in my electric bill, maybe $10/month. The Jetsetter LX is very well insulated and seems to keep hot without needing a lot of power for reheating - I do note that when I turn it down (from 96 to 80) when I am going to be away for a few weeks, it takes several days to get down to the new temp.
Of course, a lot depends on whether you have an insulating cover, how often you use it, and the ambient temperature. (Mine is on a screened porch - I started using it in February when the air temp tended to be under 20F.)