I just finished a rebuild on my 97 SPX (787 engine, did the work myself) and came in at around $1100. But my rebuild included things that I think don't apply to your 720 engine, like all new RAVE valves and housings, and a new counterbalance shaft.
The SeaDoo dealer nearest to me said they won't even work on these older 2-strokes anymore, so I'm not sure what the price would have been to have a shop do it.