Depending on concrete costs in your area and whether they can reach it with a readymix truck or have to barrow it, and how much surface prep is needed under the concrete (stripping topsoil, replacing unsuitable soil, removing tree roots, etc) rough number $600-1200. What your local readymix minimum delivery is counts too - might be 3 cubic yards when you only need a bit over one, which means you pay for three times more than you take, or even have to find a place to dump 2 yards of concrete (most readymix companies will not ship less than 3 yards to ensure mix consistency, and many charge to take unused concrete back.) So, if you are certainyou want a concrete pad, find out how much you have to take and consider a pathway or something to use up the rest, though that is more lablro and forming and rebar as well.
Check your manufacturer's recommendations - hot tubs can be laid on a timber deck, and generally on a prefab sauna and hot tub mat like EZ Pad (for $300-400) over level grass, sand or gravel, or on a gravel pad. Could cut your cost WAY down.
Also, if you ever move the hot tub might or might not be a benefit (many realtors say it is a negative to buyers - I agree, as not many people want the maintenance cost), so having a gravel pad or wood deck would be a plus, whereas a concrete pad would be a detriment. If you do go with a concrete pad you might consider the small additional cost of having it colored brown or deep red to blend in with the yard a bit more.