I have an Australian made Heritage Riverside 750 litre spa which comfortably fits 3 people, but I have had 6 people in it at times. It's definitely one of the smaller ones out there but the benefit is that it's cheap as chips to run. When I first got it I noticed no discernible difference on my power bill - If I had to put a figure on it I'd honestly say it's probably no more than around $20 a month.
Having said that, I took a tip from a pro and laid a 2 inch thick piece of polystyrene on the ground underneath it - this provides a superior amount of insulation as I am not losing heat to the ground underneath. I can't recommend this tip highly enough. The polystryrene was free - I just approached a packaging manufacturer who directed me to their rubbish pile. I found two perfectly sized pieces which I just taped together, then once the spa was placed on top I trimmed the edges to around one inch larger than the spa's base all around.