If you are like me and prefer to use your spa with the pumps running then the bills will be higher, generally motors tend not to be very efficient especially powerful ones. Some spas cost more than others to run and a big factor to this is the rating of the pumps. I have seen spas that are only rated at 13 amps these are not very powerful and should be cheaper to run.
As mentioned before try cutting down on your filter cycles and do not run the pumps all the time you are in, however some people like me do not like sitting in just still water.