Unless you put a dedicated meter on the spa, you really are only guessing at the cost. There's SO many things that can cause changes in energy usage....a second older and failing refrigerator in the garage, colder weather this year as compared to last, newer electronics (TV's) that continue to suck large amounts of power even when turned off (this is why they turn on almost instantly), and much much more. I highly doubt that 10 year old spa is costing $100 a month...though, if the cover is shot and water logged it's possible...but would happen with any spa.
There are older spas that are going to be much more energy efficient than some new spas. It's simply a matter of quality and how they're built and how they're insulated.